Adjust function definition in moea64_native.c to avoid clang 15 warning

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

sys/powerpc/aim/moea64_native.c:306:22: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
moea64_install_native()
                     ^
                      void

This is because moea64_install_native() 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 ff490346fd519501e9263988959ffb583013ccdb)