use private linkage where possible

Task-number: QTBUG-34463
Change-Id: Iffbeed2f1a27fd38b5960afa9715815ac11b722a
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/multimediawidgets/multimediawidgets.pro

@@ -35,7 +35,7 @@ maemo6 {
 
         SOURCES += qgraphicsvideoitem_maemo6.cpp
 
-        LIBS += -lX11
+        LIBS_PRIVATE += -lX11
     } else {
         SOURCES += qgraphicsvideoitem.cpp
     }