Add missing gthread-2.0 linkage.

gthread-2.0 linkage is required for g_thread_init when compiling with
glib 2.32.4.

BUG=chromium-os:34103
TEST=`emerge metrics` with glib 2.30.2 and 2.32.4

Change-Id: Id6ef58ee568989123fc563af47e2670066d01b93
Reviewed-on: https://gerrit.chromium.org/gerrit/32507


Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>