xcb: replace a use of an inefficient QList with QVector

The Transaction type is larger than a void*, so better use
a vector to hold them.

Change-Id: Ie2c00d053f67addbce6fd8107f659c56ccdd37aa
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/plugins/platforms/xcb/qxcbdrag.h

@@ -46,7 +46,6 @@
 #include <private/qsimpledrag_p.h>
 #include <qxcbobject.h>
 #include <xcb/xcb.h>
-#include <qlist.h>
 #include <qpoint.h>
 #include <qrect.h>
 #include <qsharedpointer.h>
@@ -160,7 +159,7 @@ private:
         QDrag *drag;
         QTime time;
     };
-    QList<Transaction> transactions;
+    QVector<Transaction> transactions;
 
     int transaction_expiry_timer;
     void restartDropExpiryTimer();