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/drivers/net/sandbox-raw.c b/drivers/net/sandbox-raw.c
index 7e6625d..3707ee3 100644
--- a/drivers/net/sandbox-raw.c
+++ b/drivers/net/sandbox-raw.c
@@ -9,6 +9,7 @@
 #include <asm/eth-raw-os.h>
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <malloc.h>
 #include <net.h>