Fix the build with WITHOUT_GOOGLETEST

Attempting to build the fusefs tests WITHOUT_GOOGLETEST will result in an
error if the host system or sysroot doesn't already have googletest headers
in /usr/include/private (e.g. host built/installed WITHOUT_GOOGLETEST, clean
cross-buildworld WITHOUT_GOOGLETEST).

Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D21367