env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/Synology/ds414/cmd_syno.c b/board/Synology/ds414/cmd_syno.c
index 59e6fe0..777948f 100644
--- a/board/Synology/ds414/cmd_syno.c
+++ b/board/Synology/ds414/cmd_syno.c
@@ -7,6 +7,7 @@
 
 #include <common.h>
 #include <div64.h>
+#include <env.h>
 #include <spi.h>
 #include <spi_flash.h>
 #include <linux/mtd/mtd.h>