Fix build with GTK+ < 2.20 due to undefined key code constants

Include gdk/gdkkeysyms.h from wx/gtk/private/gtk2-compat.h as it must be
included before it for the compatibility header to define the new names
for the key constants even when using old GTK+ versions.

include/wx/gtk/private/gtk2-compat.h

@@ -539,6 +539,8 @@ static inline void wx_gtk_widget_get_preferred_size(GtkWidget* widget, GtkRequis
 }
 #define gtk_widget_get_preferred_size wx_gtk_widget_get_preferred_size
 
+#include <gdk/gdkkeysyms.h>
+
 #if defined(GDK_Alt_L) && !defined(GDK_KEY_Alt_L)
 #define GDK_KEY_Alt_L GDK_Alt_L
 #define GDK_KEY_Alt_R GDK_Alt_R

src/gtk/listbox.cpp

@@ -34,8 +34,6 @@
 #include "wx/gtk/private/treeentry_gtk.h"
 #include "wx/gtk/private/treeview.h"
 
-#include <gdk/gdkkeysyms.h>
-
 //-----------------------------------------------------------------------------
 // data
 //-----------------------------------------------------------------------------

src/gtk/radiobox.cpp

@@ -19,8 +19,6 @@
 
 #include "wx/gtk/private.h"
 
-#include <gdk/gdkkeysyms.h>
-
 //-----------------------------------------------------------------------------
 // wxGTKRadioButtonInfo
 //-----------------------------------------------------------------------------

src/gtk/window.cpp

@@ -44,8 +44,6 @@
 #include "wx/private/textmeasure.h"
 using namespace wxGTKImpl;
 
-#include <gdk/gdkkeysyms.h>
-
 #ifdef GDK_WINDOWING_X11
 #include <gdk/gdkx.h>
 #include "wx/x11/private/wrapxkb.h"