Fix sim-eng build.

Appears to have been broken by:

commit 9779b221e999583ff89e0dfc40e56398737adbb3
Author: Mathias Agopian <mathias@google.com>
Date:   Mon Sep 7 16:32:45 2009 -0700

    fix [2068105] implement queueBuffer/lockBuffer/dequeueBuffer properly

For some reason we don't like to have "-lpthread" globally -- it's a no-op
on device builds, but required for many host tools and all sim binaries --
so adding the use of pthread calls requires adding the library explicitly.