sconeserver: requires thread support

Fixes autobuild failures such as the following.

  http://autobuild.buildroot.net/results/a290aa34bd44c0e08f7aa44a7c606420668eef6d/



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>