xcb: Use std::move and pass argument by rvalue reference

It will avoid a creation of temporary object and avoid copying.

Change-Id: Ifae5f6f9e36bcb07f4bacc31f151f8adcfa621a3
Reviewed-by: Anton Kudryavtsev <antkudr@mail.ru>
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforms/xcb/qxcbconnection_screens.cpp

@@ -384,7 +384,7 @@ void QXcbConnection::initializeScreens()
             }
             siblings << screen;
         }
-        virtualDesktop->setScreens(siblings);
+        virtualDesktop->setScreens(std::move(siblings));
         xcb_screen_next(&it);
         ++xcbScreenNumber;
     } // for each xcb screen

src/plugins/platforms/xcb/qxcbscreen.h

@@ -79,7 +79,7 @@ public:
     QXcbScreen *screenAt(const QPoint &pos) const;
 
     QList<QPlatformScreen *> screens() const { return m_screens; }
-    void setScreens(QList<QPlatformScreen *> sl) { m_screens = sl; }
+    void setScreens(QList<QPlatformScreen *> &&sl) { m_screens = std::move(sl); }
     void removeScreen(QPlatformScreen *s) { m_screens.removeOne(s); }
     void addScreen(QPlatformScreen *s);
     void setPrimaryScreen(QPlatformScreen *s);