Bug 922460 followup - Don't build layout/media/webrtc in disable-webrtc builds. r=ted granted over IRC

layout/media/moz.build

@@ -8,4 +8,5 @@ MODULE = 'layout'
 
 LIBRARY_NAME = 'gkmedias'
 
-DIRS += ['webrtc']
+if CONFIG['MOZ_WEBRTC']:
+    DIRS += ['webrtc']