[gtk+] gdk/gdkconfig.h.meson: Add GDK_WINDOWING_WIN32

[Chun-wei Fan <fanchunwei src gnome org>]

commit e5a1de1c1948898dda89fae4bbc17addb3595d20
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Tue Aug 1 16:37:43 2017 +0800

    gdk/gdkconfig.h.meson: Add GDK_WINDOWING_WIN32
    
    This is so that Meson can add this define once it is determined that we
    are building for Windows.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785210

 gdk/gdkconfig.h.meson |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdkconfig.h.meson b/gdk/gdkconfig.h.meson
index f832c04..115c612 100644
--- a/gdk/gdkconfig.h.meson
+++ b/gdk/gdkconfig.h.meson
@@ -13,6 +13,7 @@ G_BEGIN_DECLS
 #mesondefine GDK_WINDOWING_X11
 #mesondefine GDK_WINDOWING_BROADWAY
 #mesondefine GDK_WINDOWING_WAYLAND
+#mesondefine GDK_WINDOWING_WIN32
 #mesondefine GDK_WINDOWING_MIR
 
 #mesondefine GDK_RENDERING_VULKAN