runtime/cgo: annotate unused variable with __attribute__((unused))

Fixes #28095

Change-Id: Id8668d52986c9805213e8847f49fe42dfde2e01a
Reviewed-on: https://go-review.googlesource.com/c/140797
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/gcc_libinit.c

@@ -63,7 +63,7 @@ _cgo_wait_runtime_init_done() {
 }
 
 void
-x_cgo_notify_runtime_init_done(void* dummy) {
+x_cgo_notify_runtime_init_done(void* dummy __attribute__ ((unused))) {
 	pthread_mutex_lock(&runtime_init_mu);
 	runtime_init_done = 1;
 	pthread_cond_broadcast(&runtime_init_cond);