Fix compile error in generated thunks for throwing functions returning String

[sidepelican]

This PR fixes a compilation error in the generated JNI thunk code.

• original code

public func throwsHello() throws -> String {
    return "Hello"
}

• generated code

@_cdecl("Java_my_lib_AndroidSwift__00024throwsHello__")
public func Java_my_lib_AndroidSwift__00024throwsHello__(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass) -> jstring? {
  do {
    return try AndroidSwift.throwsHello().getJNIValue(in: environment)
  } catch {
    environment.throwAsException(error)
    return .jniPlaceholderValue // error: contextual member reference to static property 'jniPlaceholderValue' requires 'Self' constraint in the protocol extension
  }
} // printCDecl(_:javaMethodName:parentName:parameters:resultType:_:) @ JExtractSwiftLib/JNISwift2JavaGenerator+SwiftThunkPrinting.swift:725

jstring? does not have a .jniPlaceholderValue extension.
There might be other types with the same issue, but for now, I’ve only implemented a fix for String.

[ktoso]

Nice, thank you!