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

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]
    mpc745x_sleep()
                 ^
                  void

This is because mpc745x_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