gtest needs -lpthread.

Host gtests that don't use pthreads themselves need to be linked
against libpthread because gtest itself uses pthreads to ensure
thread-safety. (It can't know in advance whether the code under
test is going to spin up threads, so it has to be pessimistic.)

Change-Id: I14fb1098c163d7fe6ae9a9a0314acead4ec9383d