Revert "test_libvpx: disable pthreads in gtest"

This reverts commit 90a9900abb79fabfd44189a959d14ca677c2777a

Seems to break the Mac build:
src/include/gtest/internal/gtest-port.h:1208:: pthread_mutex_lock(&mutex_)failed with error 22
Abort trap: 6

Change-Id: Icbe31161d7c27f1b0a28d33409e7712430bbf0ae