Make xlog_internal.h includable in frontend context.

This makes unnecessary the ugly hack used to #include postgres.h in
pg_basebackup.

Based on Alvaro Herrera's patch

src/bin/pg_basebackup/streamutil.c

@@ -11,13 +11,7 @@
  *-------------------------------------------------------------------------
  */
 
-/*
- * We have to use postgres.h not postgres_fe.h here, because there's so much
- * backend-only stuff in the XLOG include files we need.  But we need a
- * frontend-ish environment otherwise.	Hence this ugly hack.
- */
-#define FRONTEND 1
-#include "postgres.h"
+#include "postgres_fe.h"
 #include "streamutil.h"
 
 #include <stdio.h>