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/nokia/rx51/rx51.c b/board/nokia/rx51/rx51.c
index 76f6ede..71ca79d 100644
--- a/board/nokia/rx51/rx51.c
+++ b/board/nokia/rx51/rx51.c
@@ -22,6 +22,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <watchdog.h>
 #include <malloc.h>
 #include <twl4030.h>