girepository: Include config.h First in All Sources

This includes config.h in all the C-sources of girepository so that we can
get the correct export directive from config.h during compile time and
therefore export the symbols as necessary, like what GLib and GTK+ is
currently doing.

https://bugzilla.gnome.org/show_bug.cgi?id=732669

giconstantinfo.c

@@ -20,6 +20,8 @@
  * Boston, MA 02111-1307, USA.
  */
 
+#include "config.h"
+
 #include <glib.h>
 #include <string.h> // memcpy