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/lib/uuid.c b/lib/uuid.c
index ca8be2c..a48e19c 100644
--- a/lib/uuid.c
+++ b/lib/uuid.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <linux/ctype.h>
 #include <errno.h>
 #include <common.h>