[gtksourceview] Add missing <windows.h> include

[Sébastien Wilmet <swilmet src gnome org>]

commit 6dc6c0bf09f54c91788f648b543f76c97820156b
Author: Kouhei Sutou <kou clear-code com>
Date:   Sat Nov 12 23:51:28 2016 +0900

    Add missing <windows.h> include
    
    It's required for building with MinGW.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=774325

 gtksourceview/gtksourceview-init.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gtksourceview/gtksourceview-init.c b/gtksourceview/gtksourceview-init.c
index 1403c7e..edefc87 100644
--- a/gtksourceview/gtksourceview-init.c
+++ b/gtksourceview/gtksourceview-init.c
@@ -37,6 +37,8 @@
 #include "gtksourcestyleschememanager.h"
 
 #ifdef G_OS_WIN32
+#include <windows.h>
+
 static HMODULE gtksourceview_dll;
 #endif