Adjust function definitions in mmu_oea64.c to avoid clang 15 warnings

Adjust function definitions in mmu_oea64.c to avoid clang 15 warnings

With clang 15, the following -Werror warnings are produced:

    sys/powerpc/aim/mmu_oea64.c:1947:12: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    moea64_init()
               ^
                void
    sys/powerpc/aim/mmu_oea64.c:3257:17: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    moea64_scan_init()
                    ^
                     void

This is because moea64_init() and moea64_scan_init() are declared with
(void) argument lists, but defined with empty argument lists. Make the
definitions match the declarations.

MFC after:	3 days