Move Trigger and TriggerDesc structs out of rel.h into a new reltrigger.h

This lets us stop including rel.h into execnodes.h, which is a widely
used header.

src/backend/access/hash/hash.c

@@ -25,6 +25,7 @@
 #include "optimizer/cost.h"
 #include "optimizer/plancat.h"
 #include "storage/bufmgr.h"
+#include "utils/rel.h"
 
 
 /* Working state for hashbuild and its callback */

src/backend/access/hash/hashutil.c

@@ -20,6 +20,7 @@
 #include "executor/execdebug.h"
 #include "storage/bufmgr.h"
 #include "utils/lsyscache.h"
+#include "utils/rel.h"
 
 
 /*