Adjust pmc_thread_descriptor_pool_drain() definition to avoid clang 15 warning

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

sys/dev/hwpmc/hwpmc_mod.c:2462:33: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
pmc_thread_descriptor_pool_drain()
                                ^
                                 void

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

MFC after: 3 days