[gtksourceview] Add missing <config.h> include

[Sébastien Wilmet <swilmet src gnome org>]

commit d6c07d063d3b4b0eaa579c5376046d8d6c02e9ae
Author: Kouhei Sutou <kou clear-code com>
Date:   Sun Nov 13 15:33:30 2016 +0900

    Add missing <config.h> include
    
    It's required for exporting symbols on Windows because
    _GTK_SOURCE_EXTERN is defined in config.h.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=774349

 gtksourceview/gtksourceregion.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gtksourceview/gtksourceregion.c b/gtksourceview/gtksourceregion.c
index f75e574..8ab5413 100644
--- a/gtksourceview/gtksourceregion.c
+++ b/gtksourceview/gtksourceregion.c
@@ -20,6 +20,10 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
 #include "gtksourceregion.h"
 
 /**