Fix has_external_partition() incompatible pointer type warnings

cdbpartition.c: In function ‘rel_has_external_partition’:
cdbpartition.c:489:32: warning: passing argument 1 of ‘has_external_partition’ from incompatible pointer type [-Wincompatible-pointer-types]
  return has_external_partition(n->rules);
                                ^
cdbpartition.c:106:13: note: expected ‘PartitionRule * {aka struct PartitionRule *}’ but argument is of type ‘List * {aka struct List *}’
 static bool has_external_partition(PartitionRule *rules);
             ^~~~~~~~~~~~~~~~~~~~~~

src/backend/cdb/cdbpartition.c

@@ -106,7 +106,7 @@ constrNodeMatch(const void *keyPtr1, const void *keyPtr2, Size keysize);
 static void
 parruleord_open_gap(Oid partid, int2 level, Oid parent,
 					int2 ruleord, int stopkey, bool closegap);
-static bool has_external_partition(PartitionRule *rules);
+static bool has_external_partition(List *rules);
 
 /*
  * Hash keys are null-terminated C strings assumed to be stably
@@ -9381,7 +9381,7 @@ findPartitionNodeEntry(PartitionNode *partitionNode, Oid partOid)
  * external partition table
  */
 static bool
-has_external_partition(PartitionRule *rules)
+has_external_partition(List *rules)
 {
 	if (rules == NULL)
 	{