Add ALTER SYSTEM to list of not replicated DDL

Author: kelvich

multimaster.c

@@ -4787,6 +4787,7 @@ static void MtmProcessUtility(Node *parsetree, const char *queryString,
 		case T_CheckPointStmt:
 		case T_ReindexStmt:
 		case T_ExplainStmt:
+		case T_AlterSystemStmt:
 			skipCommand = true;
 			break;