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/usb/gadget/f_rockusb.c b/drivers/usb/gadget/f_rockusb.c
index f3d2477..1cfeabc 100644
--- a/drivers/usb/gadget/f_rockusb.c
+++ b/drivers/usb/gadget/f_rockusb.c
@@ -6,6 +6,7 @@
  */
 #include <config.h>
 #include <common.h>
+#include <env.h>
 #include <errno.h>
 #include <malloc.h>
 #include <memalign.h>