Cast uses of return_*(), nop() and enumerator_create_empty()

As described in the previous commit, GCC 15 uses C23 by default and that
changes the meaning of such argument-less function declarations.  So
whenever we assign such a function to a pointer that expects a function
with arguments it causes an incompatible pointer type warning.  We
could define dedicated functions/callbacks whenever necessary, but this
seems like the simpler approach for now (especially since most uses of
these functions have already been cast).

src/libcharon/plugins/android_dns/android_dns_handler.c

@@ -191,7 +191,7 @@ METHOD(enumerator_t, enumerate_dns, bool,
 	VA_ARGS_VGET(args, type, data);
 	*type = INTERNAL_IP4_DNS;
 	*data = chunk_empty;
-	this->venumerate = return_false;
+	this->venumerate = (void*)return_false;
 	return TRUE;
 }