ari/ari_websockets.c: Fix ast_debug parameter type mismatch.

This is a type mismatch fix of the debugging commit
c63316eec1 made to find out why
a testsuite test was failing only on one of the continuous
integration build agents.

Change-Id: Iba34f6e87cec331f6ac80e4daff6476ea6f00a75

res/ari/ari_websockets.c

@@ -174,7 +174,7 @@ int ast_ari_websocket_session_write(struct ast_ari_websocket_session *session,
 		return -1;
 	}
 
-	ast_debug(3, "Examining ARI event (length %zu): \n%s\n", strlen(str), str);
+	ast_debug(3, "Examining ARI event (length %u): \n%s\n", (unsigned int) strlen(str), str);
 	if (ast_websocket_write_string(session->ws_session, str)) {
 		ast_log(LOG_NOTICE, "Problem occurred during websocket write, websocket closed\n");
 		return -1;