Make the order of the header file includes consistent in backend modules.

Similar to commits 7e735035f2 and dddf4cdc33, this commit makes the order
of header file inclusion consistent for backend modules.

In the passing, removed a couple of duplicate inclusions.

Author: Vignesh C
Reviewed-by: Kuntal Ghosh and Amit Kapila
Discussion: https://postgr.es/m/CALDaNm2Sznv8RR6Ex-iJO6xAdsxgWhCoETkaYX=+9DW3q0QCfA@mail.gmail.com

src/backend/access/gist/gist.c

@@ -18,15 +18,14 @@
 #include "access/gistscan.h"
 #include "catalog/pg_collation.h"
 #include "miscadmin.h"
+#include "nodes/execnodes.h"
 #include "storage/lmgr.h"
 #include "storage/predicate.h"
-#include "nodes/execnodes.h"
 #include "utils/builtins.h"
 #include "utils/index_selfuncs.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 
-
 /* non-export function prototypes */
 static void gistfixsplit(GISTInsertState *state, GISTSTATE *giststate);
 static bool gistinserttuple(GISTInsertState *state, GISTInsertStack *stack,

src/backend/access/gist/gistget.c

@@ -17,11 +17,11 @@
 #include "access/genam.h"
 #include "access/gist_private.h"
 #include "access/relscan.h"
+#include "lib/pairingheap.h"
 #include "miscadmin.h"
+#include "pgstat.h"
 #include "storage/lmgr.h"
 #include "storage/predicate.h"
-#include "pgstat.h"
-#include "lib/pairingheap.h"
 #include "utils/float.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"

src/backend/access/gist/gistutil.c

@@ -22,10 +22,9 @@
 #include "storage/indexfsm.h"
 #include "storage/lmgr.h"
 #include "utils/float.h"
-#include "utils/syscache.h"
-#include "utils/snapmgr.h"
 #include "utils/lsyscache.h"
-
+#include "utils/snapmgr.h"
+#include "utils/syscache.h"
 
 /*
  * Write itup vector to page, has no control of free space.