Adjust prototype_unload() definition to avoid clang 15 warning

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

sys/cddl/dev/prototype.c:99:17: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
prototype_unload()
                ^
                 void

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

MFC after: 3 days