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.

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