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/videomodes.c b/drivers/video/videomodes.c
index d761432..ac25b45 100644
--- a/drivers/video/videomodes.c
+++ b/drivers/video/videomodes.c
@@ -58,6 +58,7 @@
 
 #include <common.h>
 #include <edid.h>
+#include <env.h>
 #include <errno.h>
 #include <linux/ctype.h>