Add a const decoration

Useful for a subsequent patch.

Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da@eisentraut.org

src/backend/parser/parse_type.c

@@ -537,7 +537,7 @@ LookupCollation(ParseState *pstate, List *collnames, int location)
  * pstate is only used for error location purposes, and can be NULL.
  */
 Oid
-GetColumnDefCollation(ParseState *pstate, ColumnDef *coldef, Oid typeOid)
+GetColumnDefCollation(ParseState *pstate, const ColumnDef *coldef, Oid typeOid)
 {
 	Oid			result;
 	Oid			typcollation = get_typcollation(typeOid);

src/include/parser/parse_type.h

@@ -36,7 +36,7 @@ extern char *TypeNameToString(const TypeName *typeName);
 extern char *TypeNameListToString(List *typenames);
 
 extern Oid	LookupCollation(ParseState *pstate, List *collnames, int location);
-extern Oid	GetColumnDefCollation(ParseState *pstate, ColumnDef *coldef, Oid typeOid);
+extern Oid	GetColumnDefCollation(ParseState *pstate, const ColumnDef *coldef, Oid typeOid);
 
 extern Type typeidType(Oid id);