Adjust function definition in hwpmc to avoid clang 15 warning

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

sys/dev/hwpmc/hwpmc_arm64.c:530:21: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
pmc_arm64_initialize()
                    ^
                     void

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

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 05cef74742154cf3c89be214a464d8fabddf8d7d)