Fix inclusions of postgres_fe.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 postgres_fe.h.

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

src/interfaces/ecpg/ecpglib/extern.h

@@ -3,12 +3,12 @@
 #ifndef _ECPG_LIB_EXTERN_H
 #define _ECPG_LIB_EXTERN_H
 
-#include "postgres_fe.h"
 #include "libpq-fe.h"
 #include "sqlca.h"
 #include "sqlda-native.h"
 #include "sqlda-compat.h"
 #include "ecpg_config.h"
+
 #ifndef CHAR_BIT
 #include <limits.h>
 #endif

src/interfaces/ecpg/test/pg_regress_ecpg.c

@@ -16,9 +16,12 @@
  *-------------------------------------------------------------------------
  */
 
+#include "postgres_fe.h"
+
 #include "pg_regress.h"
 
 #define LINEBUFSIZE 300
+
 static void
 ecpg_filter(const char *sourcefile, const char *outfile)
 {

src/interfaces/libpq/libpq-int.h

@@ -21,7 +21,6 @@
 #define LIBPQ_INT_H
 
 /* We assume libpq-fe.h has already been included. */
-#include "postgres_fe.h"
 #include "libpq-events.h"
 
 #include <time.h>