Update headers to cause warnings on unused returns and null args.

In places where an ignored return or a null-arg is a sure indication
of a bug add the GCC warning attributes. The null arg annotation
is not enable for Opus itself because it will cause the compiler
to optimize out some null checks. I don't trust our callers
quite that much.

tests/test_opus_api.c

@@ -1382,7 +1382,7 @@ int test_repacketizer_api(void)
  * handling in our codebase, and the lack of thread saftey isn't an
  * issue here. We therefore disable the warning for this function.
  */
-#if __GNUC_PREREQ(4,6)
+#if OPUS_GNUC_PREREQ(4,6)
 /* Save the current warning settings */
 #pragma GCC diagnostic push
 #endif