Fix struct vs. class mismatch in forward definitions

The warnings were being hidden by the use of -isystem to include
frameworks/native/include.

Bug: 31752268
Test: m -j
Change-Id: Ib7227295e7b94c79cdc52e19c71be4066a11cedc

media/jni/android_media_MediaCodec.h

@@ -31,7 +31,7 @@ struct ALooper;
 struct AMessage;
 struct AString;
 struct ICrypto;
-struct IGraphicBufferProducer;
+class IGraphicBufferProducer;
 struct MediaCodec;
 struct PersistentSurface;
 class Surface;