env: Move env_get() to env.h

Move env_get() 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/serial/usbtty.c b/drivers/serial/usbtty.c
index d0465b8..76d9c8a 100644
--- a/drivers/serial/usbtty.c
+++ b/drivers/serial/usbtty.c
@@ -10,6 +10,7 @@
 #include <common.h>
 #include <config.h>
 #include <circbuf.h>
+#include <env.h>
 #include <stdio_dev.h>
 #include <asm/unaligned.h>
 #include "usbtty.h"