sconeserver: needs zlib

Since bump to 6b932d7d8dbb700b830205e7111e469cefff490c and
https://github.com/sconemad/sconeserver/commit/34b01e9d332f912da82ab1c09529a33969ff6ebb,
sconeserver needs zlib so select it

Fixes:
 - http://autobuild.buildroot.org/results/b5e2b8635b37e6f2803a6031b154cf67685a4221



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>