codec: Fix `pragma GCC diagnostic pop` missing corresponding push

src/video_core/command_classes/codecs/codec.h

@@ -10,6 +10,7 @@
 
 extern "C" {
 #if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wconversion"
 #endif
 #include <libavcodec/avcodec.h>