Add gcc-style annotations for printf-like functions.

Bug: None
Change-Id: I37a553d254cb61a882b98b14274c0fdfba039992
Reviewed-on: https://webrtc-review.googlesource.com/33002
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21281}

rtc_base/testutils.h

@@ -175,6 +175,10 @@ public:
   void QueueString(const char* data) {
     QueueData(data, strlen(data));
   }
+#if defined(__GNUC__)
+  // Note: Implicit |this| argument counts as the first argument.
+  __attribute__((__format__(__printf__, 2, 3)))
+#endif
   void QueueStringF(const char* format, ...) {
     va_list args;
     va_start(args, format);