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/mx3fb.c b/drivers/video/mx3fb.c
index 176ae70..a984443 100644
--- a/drivers/video/mx3fb.c
+++ b/drivers/video/mx3fb.c
@@ -6,6 +6,7 @@
  * HALE electronic GmbH, <helmut.raiger@hale.at>
  */
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <video_fb.h>