fix implicit-fallthrough warnings under clang

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/player/Thread.cxx

@@ -1176,7 +1176,9 @@ try {
 			}
 
 			/* fall through */
-			gcc_fallthrough;
+#if CLANG_OR_GCC_VERSION(7,0)
+			[[fallthrough]];
+#endif
 
 		case PlayerCommand::PAUSE:
 			next_song.reset();