gdkdisplay-win32.c: Define _WIN32_WINNT

... to be for Vista (0x0600) or later.  This is so that the necessary
items in the Windows headers be activated so that the code will build
properly on mingw-w64, and we already require Vista or later for GTK+.

Thanks Ting-Wei Lan for pointing this out.

See: https://bugzilla.gnome.org/show_bug.cgi?id=768081#c62

gdk/win32/gdkdisplay-win32.c

@@ -17,6 +17,9 @@
  */
 
 #include "config.h"
+
+#define _WIN32_WINNT 0x0600
+
 #include "gdk.h"
 #include "gdkprivate-win32.h"
 #include "gdkdisplay-win32.h"