package/libs/zlib: Add host build

package/libs/zlib: Add host build

Some packages such as Python/Python3 (host pip/pip3) needs this
to compile.

More detailed explanation provided by Alexandru:

"i need the zlib/host for Python/Python3 ; because, it seems the
host pip/pip3 needs this to work ; i suspect in older versions
this worked, because some of the host's build env would be used
in the build, and then the zlib-dev from the host distro would
be used ; now, the host-build does not seem to have any
-I/usr/include stuff, which is good

and it also seems that Python/Python3 does not like it if the
zlib-dev package is too old, so using this zlib/host would be
good for this as well"

Source:
https://github.com/lede-project/source/pull/1329#issuecomment-351055861

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>