QWidget: Fix unused parameter warning when there's no Graphics View.

This reduces the amount of repetitive compiler warnings.

https://github.com/ariya/phantomjs/issues/11880

src/qt/src/gui/kernel/qwidget_p.h

@@ -611,6 +611,8 @@ public:
                 }
             }
         }
+#else
+        Q_UNUSED(widget);
 #endif
         return screen;
     }