modules_k/dispatcher: include stddef.h to let sip-router compile on old distros

modules_k/dispatcher/ds_ht.c

@@ -20,6 +20,7 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#include <stddef.h>
 #include <regex.h>
 
 #include "../../mem/shm_mem.h"