libtorrent: zlib is mandatory

zlib isn't optional even though it seems like it, fixes build failure
such as:

log.cc:51:18: fatal error: zlib.h: No such file or directory

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>