Adjust function definition in ofw to avoid clang 15 warning

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

sys/dev/ofw/openfirm.c:826:9: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
OF_enter()
        ^
         void

This is because OF_enter() and OF_exit are declared with a (void)
argument list, but defined with an empty argument list. Make the
definition match the declaration.

Sponsored by: The FreeBSD Foundation