Fix compilation with gcc 4.8

GCC 4.8 doesn't like QPointers in signal connections.
Also, Default Member Initializers for aggregates
are C++14, so we need to write a constructor to make
a class with DMIs work in C++11.

Change-Id: I567c60a8081fa2f268517ec747bd6534674bcce5
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/plugins/platforms/wayland/qwaylanddisplay_p.h

@@ -195,6 +195,11 @@ private:
     void requestWaylandSync();
 
     struct Listener {
+        Listener() = default;
+        Listener(RegistryListener incomingListener,
+                 void* incomingData)
+        : listener(incomingListener), data(incomingData)
+        {}
         RegistryListener listener = nullptr;
         void *data = nullptr;
     };