dm: blk: Fix warning on !CONFIG_BLK

When we don't have CONFIG_BLK defined we don't have a forward
declaration of struct udevice, and thus get a warning about it on
blk_get_from_parent(), which we only have when CONFIG_BLK is set.  Move
the declaration of blk_get_from_parent() to be with the other CONFIG_BLK
parts.

Fixes 9f103b9c ("dm: blk: Add a way to obtain a block device from ...")
Signed-off-by: Tom Rini <trini@konsulko.com>