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

ginvoke.c

@@ -19,6 +19,8 @@
  * Boston, MA 02111-1307, USA.
  */
 
+#include "config.h"
+
 #include <stdlib.h>
 
 #include <glib.h>
@@ -26,7 +28,6 @@
 
 #include <girepository.h>
 #include "girffi.h"
-#include "config.h"
 
 /**
  * value_to_ffi_type: