Make WARN_UNUSED_RESULT a no-op on gcc

Because on gcc, cast to void doesn't silence the warning. See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

Also add an RTC_ prefix to the macro instead of only defining it if it
wasn't already defined, to ensure that we always get our own version.

BUG=none

Review-Url: https://codereview.webrtc.org/2797983003
Cr-Commit-Position: refs/heads/master@{#17563}

webrtc/modules/audio_coding/codecs/ilbc/decode.h

@@ -33,6 +33,6 @@ int WebRtcIlbcfix_DecodeImpl(
                                            structure */
     int16_t mode               /* (i) 0: bad packet, PLC,
                                       1: normal */
-    ) WARN_UNUSED_RESULT;
+    ) RTC_WARN_UNUSED_RESULT;
 
 #endif