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/private/media/AudioTrackShared.h b/include/private/media/AudioTrackShared.h
index 7becf57..9da5ef3 100644
--- a/include/private/media/AudioTrackShared.h
+++ b/include/private/media/AudioTrackShared.h
@@ -172,7 +172,7 @@
                 // client write-only, server read-only
                 uint16_t    mSendLevel;      // Fixed point U4.12 so 0x1000 means 1.0
 
-                uint16_t    mPad2;           // unused
+                uint16_t    mPad2 __attribute__((__unused__)); // unused
 
                 // server write-only, client read
                 ExtendedTimestampQueue::Shared mExtendedTimestampQueue;