freebsd: Fix building on FreeBSD
These changes are necessary to build on FreeBSD, but not sufficient. Since FreeBSD does not have systemd nor linux-pam, the following MRs are also required. But those might be required on other non-systemd or non-Linux systems as well.
A branch with all changes required to build is available here: https://gitlab.freedesktop.org/JohnAZoidberg/fprintd/-/tree/freebsd It builds with:
meson -Dlibsystemd=basu -Dsystemd=false -Dopenpam=true build
ninja -C build