Split index xlog headers from other private index headers.

The xlog-specific headers need to be included in both frontend code -
specifically, pg_waldump - and the backend, but the remainder of the
private headers for each index are only needed by the backend.  By
splitting the xlog stuff out into separate headers, pg_waldump pulls
in fewer backend headers, which is a good thing.

Patch by me, reviewed by Michael Paquier and Andres Freund, per a
complaint from Dilip Kumar.

Discussion: http://postgr.es/m/CA+TgmoZ=F=GkxV0YEv-A8tb+AEGy_Qa7GSiJ8deBKFATnzfEug@mail.gmail.com

src/backend/access/gist/gistbuild.c

@@ -18,6 +18,7 @@
 
 #include "access/genam.h"
 #include "access/gist_private.h"
+#include "access/gistxlog.h"
 #include "access/xloginsert.h"
 #include "catalog/index.h"
 #include "miscadmin.h"

src/backend/access/gist/gistxlog.c

@@ -15,6 +15,7 @@
 
 #include "access/bufmask.h"
 #include "access/gist_private.h"
+#include "access/gistxlog.h"
 #include "access/xloginsert.h"
 #include "access/xlogutils.h"
 #include "utils/memutils.h"