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/drivers/usb/gadget/f_sdp.c b/drivers/usb/gadget/f_sdp.c
index fab7ce6..bcd1c5d 100644
--- a/drivers/usb/gadget/f_sdp.c
+++ b/drivers/usb/gadget/f_sdp.c
@@ -19,6 +19,7 @@
 #include <errno.h>
 #include <common.h>
 #include <console.h>
+#include <env.h>
 #include <malloc.h>
 
 #include <linux/usb/ch9.h>