package/libplist: needs threads

Upstream added a mandatory pthread check:
https://github.com/libimobiledevice/libplist/commit/eec2e855b4f192cb1808d1f02b6bc8935a979025

Fixes
http://autobuild.buildroot.net/results/863/863bb43db222f8e63d60b1fc4a53299575727de1/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>