Add using declaration for ctors to generic wxImageList

Instead of explicitly forwarding the ctors to the base class manually,
just use them.

include/wx/generic/imaglist.h

@@ -81,12 +81,7 @@ class WXDLLIMPEXP_CORE wxImageList: public wxGenericImageList
     wxDECLARE_DYNAMIC_CLASS(wxImageList);
 
 public:
-    wxImageList() {}
-
-    wxImageList( int width, int height, bool mask = true, int initialCount = 1 )
-        : wxGenericImageList(width, height, mask, initialCount)
-    {
-    }
+    using wxGenericImageList::wxGenericImageList;
 };
 #endif // !wxHAS_NATIVE_IMAGELIST