Adjust dtrace_unload() definition to avoid clang 15 warning

Adjust dtrace_unload() definition to avoid clang 15 warning

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

    In file included from sys/cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c:18440:
    sys/cddl/dev/dtrace/dtrace_unload.c:26:14: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    dtrace_unload()
                 ^
                  void

This is because dtrace_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