Avoid including postgres.h in frontend compiles of src/port.

This isn't kosher, and doesn't play nicely with my recent changes to the
Makefile in this directory.

src/port/dirent.c

@@ -13,7 +13,12 @@
  *-------------------------------------------------------------------------
  */
 
+#ifndef FRONTEND
 #include "postgres.h"
+#else
+#include "postgres_fe.h"
+#endif
+
 #include <dirent.h>
 
 

src/port/pipe.c

@@ -15,7 +15,11 @@
  *-------------------------------------------------------------------------
  */
 
+#ifndef FRONTEND
 #include "postgres.h"
+#else
+#include "postgres_fe.h"
+#endif
 
 #ifdef WIN32
 int

src/port/win32error.c

@@ -11,7 +11,11 @@
  *-------------------------------------------------------------------------
  */
 
+#ifndef FRONTEND
 #include "postgres.h"
+#else
+#include "postgres_fe.h"
+#endif
 
 static const struct
 {