synology: Include env.h to permit reading the environment

This file reads from the environment but does not include the correct
header. Update it.

Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tony Dinh <mibodhi@gmail.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

board/Synology/ds414/ds414.c

@@ -4,6 +4,7 @@
  * Copyright (C) 2015 Phil Sutter <phil@nwl.cc>
  */
 
+#include <env.h>
 #include <init.h>
 #include <miiphy.h>
 #include <asm/global_data.h>