Fix inclusions of c.h from .h files.

We have a project policy that every .c file should start by including
postgres.h, postgres_fe.h, or c.h as appropriate; and then there is no
need for any .h file to explicitly include any of these.  Fix a few
headers that were violating this policy by including c.h.

Discussion: https://postgr.es/m/CAEepm=2zCoeq3QxVwhS5DFeUh=yU6z81pbWMgfOB8OzyiBwxzw@mail.gmail.com
Discussion: https://postgr.es/m/11634.1488932128@sss.pgh.pa.us

src/bin/pg_rewind/fetch.h

@@ -15,8 +15,6 @@
 #ifndef FETCH_H
 #define FETCH_H
 
-#include "c.h"
-
 #include "access/xlogdefs.h"
 
 #include "filemap.h"

src/bin/pg_rewind/pg_rewind.h

@@ -11,8 +11,6 @@
 #ifndef PG_REWIND_H
 #define PG_REWIND_H
 
-#include "c.h"
-
 #include "datapagemap.h"
 
 #include "access/timeline.h"