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/api/api.c b/api/api.c
index 9f03f1a..8a41b7c 100644
--- a/api/api.c
+++ b/api/api.c
@@ -8,6 +8,7 @@
 #include <config.h>
 #include <command.h>
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <environment.h>
 #include <linux/types.h>