Adjust pcm_md_initialize() definition to avoid clang 15 warning

With clang 15, the following -Werror warning is produced:

sys/dev/hwpmc/hwpmc_x86.c:245:18: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
pcm_md_initialize()
                 ^
                  void

This is because pcm_md_initialize() is declared with a (void) argument
list, but defined with an empty argument list. Make the definition match
the declaration.

MFC after: 3 days

(cherry picked from commit dc0cde7a21a6aab0a5e876d8f890764e32718932)