Fix trivial warnings hidden by -isystem

Fix the trivial warnings that are hidden by the use of -isystem to
include frameworks/av and caught by -Werror.

Test: m -j checkbuild
Bug: 31751828
Change-Id: I155f9f772ce0a84b364313814cb7cf528b9de4eb
diff --git a/include/media/CasImpl.h b/include/media/CasImpl.h
index 0dd506c..726f1ce 100644
--- a/include/media/CasImpl.h
+++ b/include/media/CasImpl.h
@@ -27,7 +27,7 @@
 using namespace media;
 using namespace MediaCas;
 using binder::Status;
-class CasPlugin;
+struct CasPlugin;
 class SharedLibrary;
 
 class CasImpl : public BnCas {