libmemcached: fix linking with libpthread

Add patch to fix linking against libpthread (from Fedora).

https://bugzilla.redhat.com/show_bug.cgi?id=1037707
https://bugs.launchpad.net/libmemcached/+bug/1281907

This fixes (upcoming) libmemcached use in collectd, which currently
doesn't work because of link failure caused by missing "pthread_once"
symbol.