selftests/futex: don't pass a const char* to asprintf(3)

When building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...clang issues this warning:

futex_requeue_pi.c:403:17: warning: passing 'const char **' to parameter
of type 'char **' discards qualifiers in nested pointer types
[-Wincompatible-pointer-types-discards-qualifiers]

This warning fires because test_name is passed into asprintf(3), which
then changes it.

Fix this by simply removing the const qualifier. This is a local
automatic variable in a very short function, so there is not much need
to use the compiler to enforce const-ness at this scope.

[1] https://lore.kernel.org/all/20240329-selftests-libmk-llvm-rfc-v1-1-2f9ed7d1c49f@valentinobst.de/

Fixes: f17d8a87

 ("selftests: fuxex: Report a unique test name per run of futex_requeue_pi")
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>