glib tests: add pthread flag to 'thread' test

Commit 99bdfd1b introduced a direct call to pthreads_setname_np in the
'thread' test case.  Because we are directly calling pthreads functions
from this file now, we need to make sure we link it with the system
thread library flags (as we already do for another file).

https://bugzilla.gnome.org/show_bug.cgi?id=765712
1 file changed