devel/googletest: require libpthread

As noted by @asomers in rS300906 (cddl/usr.sbin/zfsd/tests/Makefile
specifically), googletest requires libpthread in order to function, if
-DGTEST_HAS_PTHREAD is defined.

While this is true, the code was not being linked against libpthread,
resulting in libgtest being broken, and thus all tests reliant on the library
broken at runtime.

Adding LIBS+= -lpthread unbreaks the tests that use pthread support
internally.

PR: 236169
Reviewed by: jbeich
Approved by: jbeich (maintainer)
Differential Revision: https://reviews.freebsd.org/D19463