package/refpolicy: make sure xmllint is used

When parsing and adding modules the refpolicy build system checks their
validity using xmllint. By default the host system version is used and
if not found an error is displayed but the build is not stopped. This
leads to interesting issues where modules are not added correctly to
modules.conf[1] (other possible issues are likely).

Fix this by adding a dependency on host-libxml2 and explicitly use the
xmllint binary built by Buildroot.

[1] https://lore.kernel.org/buildroot/20210830114531.2285178-1-jose.pekkarinen@unikie.com/



Tested-by: José Pekkarinen <jose.pekkarinen@unikie.com>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>