Check for just GTK+ 2.0 before including gtk2-compat.h

This doesn't really change anything but makes the intention of the code more
clear: this check is needed not because things don't work with GTK+ < 2.14 but
because gtk2-compat.h is for GTK+ 2 only, while this source file is also used
for GTK+1.

src/unix/utilsx11.cpp

@@ -39,7 +39,7 @@
 #include <gtk/gtk.h>
 #ifdef GDK_WINDOWING_X11
 #include <gdk/gdkx.h>
-#if GTK_CHECK_VERSION(2,14,0)
+#ifdef __WXGTK20__
 #include "wx/gtk/private/gtk2-compat.h"     // gdk_window_get_screen()
 #endif
 #endif