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/net/wol.c b/net/wol.c
index 946bd91..5154334 100644
--- a/net/wol.c
+++ b/net/wol.c
@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <net.h>
 #include <environment.h>
 #include "wol.h"