Duplicate `const` specifier

[DanRStevens]

When building on Ubuntu 20.04, with #2 and #3 applied, build fails with:

error: duplicate 'const' declaration specifier [-Werror,-Wduplicate-decl-specifier]

I believe this comes from src/libxlibtrace-print.h, where the parameter __REALTYPE__(safetype) const * value is used in several places. If __REALTYPE__(safetype) is not a pointer type, this can result in both east-const and west-const being specified, resulting in a duplicate specifier.

Unfortunately removing const from the parameter seems to cause other problems. I suspect this is due to pointer types, where the different positions for const would cause one to be applied to the pointer itself, rather than the pointed to type.

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This is distinct from the change in #3.

[DanRStevens]

I re-tested this. It seems the code builds fine on Ubnutu 20.04 using GCC, but fails with the above error using Clang. During my previous test, I had an environment variable set that caused the build to use Clang.

export CC=clang  # Was set in my environment, leading to error