util/Compiler.h: add gcc_fallthrough

Works around build failures with ccache which may feed processed code
to GCC, which doesn't have the "fall through" code comments.

src/player/Thread.cxx

@@ -46,6 +46,7 @@
 #include "CrossFade.hxx"
 #include "tag/Tag.hxx"
 #include "Idle.hxx"
+#include "util/Compiler.h"
 #include "util/Domain.hxx"
 #include "thread/Name.hxx"
 #include "Log.hxx"
@@ -1171,6 +1172,7 @@ try {
 			}
 
 			/* fall through */
+			gcc_fallthrough;
 
 		case PlayerCommand::PAUSE:
 			next_song.reset();