devel/libvirt: fix build on -CURRENT

The src commit 5e04571cf3cf made sched.h types such as cpu_set_t
visible without the -D_WITH_CPU_SET_T. This enables building
glibc-compatible code, but fails as some of the macros are missing on
the FreeBSD side. Temporarily fix that by disabling building this
glibc compatible code on non-Linux.