Fix test failure on wsrep/variables test case.

mysql-test/include/galera_have_debug_sync.inc

+--disable_query_log
+
+--let $galera_have_debug_sync = `SELECT 1 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_debug_sync_waiters'`
+
+--if (!$galera_have_debug_sync) {
+	--skip Test requires Galera debug library with debug_sync functionality
+}
+
+--enable_query_log

mysql-test/suite/wsrep/t/variables.test

 --source include/have_wsrep.inc
+--source include/galera_have_debug_sync.inc
 
 SET @wsrep_provider_options_saved= @@global.wsrep_provider_options;
 SET @wsrep_cluster_address_saved= @@global.wsrep_cluster_address;