Adjust dtrace_getf_barrier() definition to avoid clang 15 warning

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

    sys/cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c:17019:20: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    dtrace_getf_barrier()
		       ^
			void

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

MFC after: 3 days