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/dfu/dfu.c b/drivers/dfu/dfu.c
index 3189495..d2b67b1 100644
--- a/drivers/dfu/dfu.c
+++ b/drivers/dfu/dfu.c
@@ -7,6 +7,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <errno.h>
 #include <malloc.h>
 #include <mmc.h>