Fix declaration of env parameter in fromjstring/tojstring functions.

[aclemons]

These should take a pointer to a JNIEnv. These were introduced in b178aa0 to
replace:

static char* utf2ucs(const char *utf8, char *ucs, size_t n);
static char* ucs2utf(const char *ucs, char *utf8, size_t n);

They were added as:

static char* fromjstring(const JNIEnv *env, jstring value);
static jstring tojstring(const JNIEnv *env, const char* value);

In this case however, we actually just wanted to declare these as a
constant pointer not a pointer to a constant.

Newer gcc versions complain with:

org_gnu_readline_Readline.c: In function ‘fromjstring’:
org_gnu_readline_Readline.c:780:54: warning: passing argument 1 of ‘((const struct JNINativeInterface_ *)*env)->GetObjectClass’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  780 |   const jclass jstringClass = (*env)->GetObjectClass(env, value);
      |                                                      ^~~
org_gnu_readline_Readline.c:780:54: note: expected ‘const struct JNINativeInterface_ **’ but argument is of type ‘const struct JNINativeInterface_ * const*’
org_gnu_readline_Readline.c:781:58: warning: passing argument 1 of ‘((const struct JNINativeInterface_ *)*env)->GetMethodID’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  781 |   const jmethodID getBytesMethodId = (*env)->GetMethodID(env, jstringClass, "getBytes", "()[B");
      |                                                          ^~~
org_gnu_readline_Readline.c:781:58: note: expected ‘const struct JNINativeInterface_ **’ but argument is of type ‘const struct JNINativeInterface_ * const*’
org_gnu_readline_Readline.c:783:74: warning: passing argument 1 of ‘((const struct JNINativeInterface_ *)*env)->CallObjectMethod’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  783 |   const jbyteArray jstringJBytes = (jbyteArray) (*env)->CallObjectMethod(env, value, getBytesMethodId);
      |                                                                          ^~~

Newer clang versions complain with:

org_gnu_readline_Readline.c:780:54: warning: passing 'const JNIEnv *' (aka 'const struct JNINativeInterface_ *const *') to parameter of type 'JNIEnv *' (aka 'const struct JNINativeInterface_ **') discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
  780 |   const jclass jstringClass = (*env)->GetObjectClass(env, value);
      |                                                      ^~~
org_gnu_readline_Readline.c:781:58: warning: passing 'const JNIEnv *' (aka 'const struct JNINativeInterface_ *const *') to parameter of type 'JNIEnv *' (aka 'const struct JNINativeInterface_ **') discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
  781 |   const jmethodID getBytesMethodId = (*env)->GetMethodID(env, jstringClass, "getBytes", "()[B");
      |                                                          ^~~
org_gnu_readline_Readline.c:783:74: warning: passing 'const JNIEnv *' (aka 'const struct JNINativeInterface_ *const *') to parameter of type 'JNIEnv *' (aka 'const struct JNINativeInterface_ **') discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
  783 |   const jbyteArray jstringJBytes = (jbyteArray) (*env)->CallObjectMethod(env, value, getBytesMethodId);
      |                                                                          ^~~

Related to #41