[gtk/wip/matthiasc/focus2] Add a missing include

[Matthias Clasen <matthiasc src gnome org>]

commit 51f3a7193c4ffcd9ed71d827199297fb57081c48
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Mar 4 19:00:30 2019 -0500

    Add a missing include
    
    If we don't include config.h, GDK_AVAILABLE_...
    does not have the desired effect, and your symbols
    are not exported. And you spend hours tearing out
    your hair.

 gtk/gtkwidgetfocus.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtk/gtkwidgetfocus.c b/gtk/gtkwidgetfocus.c
index 230f76d19e..a380295798 100644
--- a/gtk/gtkwidgetfocus.c
+++ b/gtk/gtkwidgetfocus.c
@@ -15,6 +15,8 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "config.h"
+
 #include "gtkwidgetprivate.h"
 
 typedef struct _CompareInfo CompareInfo;