gsk: Include config.h

The more you know:
Not including config.h causes the public API implemented in this file to
not be exported in the resulting library.

gsk/gskrendernodeimpl.c

@@ -16,6 +16,8 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "config.h"
+
 #include "gskrendernodeprivate.h"
 
 #include "gskcairoblurprivate.h"