freebsd/netlink: use new libc branch following 0.2

freebsd/netlink: use new libc branch following 0.2

Update of crate rustix now wants libc 0.2.152, which lives on the
libc-0.2 branch while the main libc branch sails towards 0.3.  To be
able to continue building until the stuff we need gets released as
0.3, without causing our PRs to become unmergeable, a new
freebsd-netlink-0.2 was necessary.

Signed-off-by: Yann Dirson <yann.dirson@vates.fr>