Fix compiler warning on MSVC

MSVC does not understand that ereport(ERROR) does not return, so just
return the first enum PartitionStrategy value to keep the compiler from
complaining about the missing return.

Discussion: https://postgr.es/m/20221104161934.GB16921@telsasoft.com

src/backend/parser/gram.y

@@ -18428,10 +18428,13 @@ parsePartitionStrategy(char *strategy)
 		return PARTITION_STRATEGY_RANGE;
 	else if (pg_strcasecmp(strategy, "hash") == 0)
 		return PARTITION_STRATEGY_HASH;
+
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("unrecognized partitioning strategy \"%s\"",
 					strategy)));
+	return PARTITION_STRATEGY_LIST;		/* keep compiler quiet */
+
 }
 
 /*