[gtk/window-api-cleanup] win32: Add a missing include

[Matthias Clasen <matthiasc src gnome org>]

commit 3600d129bf0d9a64697e123b358416e524e19005
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Mar 27 18:47:56 2019 -0400

    win32: Add a missing include
    
    We are using round() now, so include math.h.

 gdk/win32/gdkdevice-win32.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gdk/win32/gdkdevice-win32.c b/gdk/win32/gdkdevice-win32.c
index 8d42f5389e..bea82f991f 100644
--- a/gdk/win32/gdkdevice-win32.c
+++ b/gdk/win32/gdkdevice-win32.c
@@ -21,6 +21,7 @@
 
 #include <windowsx.h>
 #include <objbase.h>
+#include <math.h>
 
 #include "gdkdevice-win32.h"
 #include "gdkwin32.h"