libubootenv: Depend on zlib

The libubootenv depends on zlib as it calls at least crc32() from
there and links against it. Add the DEPENDS entry.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
(cherry picked from commit db513f9ec59b7ac526b2cdc42b0eb2573e134bc4)
Signed-off-by: Steve Sakoman <steve@sakoman.com>