Minimal portability fix for commit e1551f96e.

Older gcc versions are not happy with having multiple declarations
for the same typedef name (not struct name).  I'm a bit dubious
as to how well-thought-out that patch was at all, but for the moment
just fix it enough so I can get some work done today.

Discussion: https://postgr.es/m/20191218101338.GB325369@paquier.xyz

src/include/parser/parse_utilcmd.h

@@ -16,7 +16,8 @@
 
 #include "parser/parse_node.h"
 
-typedef struct AttrMap AttrMap;
+struct AttrMap;					/* avoid including attmap.h here */
+
 
 extern List *transformCreateStmt(CreateStmt *stmt, const char *queryString);
 extern List *transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
@@ -30,7 +31,7 @@ extern PartitionBoundSpec *transformPartitionBound(ParseState *pstate, Relation
 												   PartitionBoundSpec *spec);
 extern IndexStmt *generateClonedIndexStmt(RangeVar *heapRel,
 										  Relation source_idx,
-										  const AttrMap *attmap,
+										  const struct AttrMap *attmap,
 										  Oid *constraintOid);
 
 #endif							/* PARSE_UTILCMD_H */

src/include/rewrite/rewriteManip.h

@@ -16,8 +16,9 @@
 
 #include "nodes/parsenodes.h"
 
+struct AttrMap;					/* avoid including attmap.h here */
+
 
-typedef struct AttrMap AttrMap;
 typedef struct replace_rte_variables_context replace_rte_variables_context;
 
 typedef Node *(*replace_rte_variables_callback) (Var *var,
@@ -72,7 +73,7 @@ extern Node *replace_rte_variables_mutator(Node *node,
 
 extern Node *map_variable_attnos(Node *node,
 								 int target_varno, int sublevels_up,
-								 const AttrMap *attno_map,
+								 const struct AttrMap *attno_map,
 								 Oid to_rowtype, bool *found_whole_row);
 
 extern Node *ReplaceVarsFromTargetList(Node *node,