env: Move env_set_hex() to env.h

Move env_set_hex() over to the new header file along with env_set_addr()
which uses it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/fs/fs.c b/fs/fs.c
index 48d8f1f..d8a4ced 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -6,6 +6,7 @@
 #include <config.h>
 #include <errno.h>
 #include <common.h>
+#include <env.h>
 #include <mapmem.h>
 #include <part.h>
 #include <ext4fs.h>