rename include/zlib.h to include/u-boot/zlib.h

Some systems have zlib.h installed in /usr/include/. This isn't the
desired file for u-boot code - we want the one in include/zlib.h.
This rename will avoid the conflict.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>