Cleanup: Use QThread::msleep to sleep instead of select

The old code used select with all-zero parameters and a timeval struct
configured to wait for 50ms. This can be drastically simplified by just
calling QThread::msleep which makes the intention much more clear.

Change-Id: I115b56ae67ae2c2cd03354e16a7bfdf56f8c15b8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/plugins/platforms/xcb/qxcbclipboard.cpp

@@ -899,10 +899,7 @@ xcb_generic_event_t *QXcbClipboard::waitForClipboardEvent(xcb_window_t win, int
         connection()->flush();
 
         // sleep 50 ms, so we don't use up CPU cycles all the time.
-        struct timeval usleep_tv;
-        usleep_tv.tv_sec = 0;
-        usleep_tv.tv_usec = 50000;
-        select(0, 0, 0, 0, &usleep_tv);
+        QThread::msleep(50);
     } while (timer.elapsed() < timeout);
 
     return 0;