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/video/mxsfb.c b/drivers/video/mxsfb.c
index 6c9a7c0..6922a13 100644
--- a/drivers/video/mxsfb.c
+++ b/drivers/video/mxsfb.c
@@ -6,6 +6,7 @@
  */
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <linux/errno.h>
 #include <malloc.h>
 #include <video.h>