test_libvpx: disable pthreads in gtest

currently threading is internal to libvpx so thread safety is unneeded
in libgtest -- visual studio builds already operate in this way as they
do not have pthread.h available by default.

this removes an unconditional link to libpthread using $(extralibs)
should libvpx require it.

Change-Id: Ieae1d693406653a54b54fba818c598836797d33b