Some additions to the 12-bit patch.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13524 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/defs.h

@@ -1880,6 +1880,7 @@ typedef struct _GdkColor        GdkColor;
 typedef struct _GdkColormap     GdkColormap;
 typedef struct _GdkFont         GdkFont;
 typedef struct _GdkGC           GdkGC;
+typedef struct _GdkVisual       GdkVisual;
 #ifdef __WXGTK20__
 typedef struct _GdkDrawable     GdkWindow;
 typedef struct _GdkDrawable     GdkBitmap;