Adjust function definition in aim_machdep.c to avoid clang 15 warning

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

sys/powerpc/aim/aim_machdep.c:750:14: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
cpu_sleep()
             ^
              void

This is because cpu_sleep() 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 06fce030e79ecce57365bc537e38ee0f25861d70)