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/common/usb_kbd.c b/common/usb_kbd.c
index cc99c6b..3873737 100644
--- a/common/usb_kbd.c
+++ b/common/usb_kbd.c
@@ -9,6 +9,7 @@
 #include <common.h>
 #include <console.h>
 #include <dm.h>
+#include <env.h>
 #include <errno.h>
 #include <malloc.h>
 #include <memalign.h>