moviplayer: suppress unused variable warning

Origin commit data
------------------
Commit: 597b40664f
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2017-09-23 (Sat, 23 Sep 2017)

src/gui/movieplayer.cpp

@@ -2844,6 +2844,8 @@ void CMoviePlayerGui::makeScreenShot(bool autoshot, bool forcover)
 		}
 	}
 	sc->Start();
+#else
+	(void)forcover;
 #endif
 	if (autoshot)
 		autoshot_done = true;