mirage-xen: depend on conf-pkg-config

The conf-pkg-config has more depext information than the mirage-xen
package has, and is likely to be kept up to date.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #36 from djs55/mirage-xen-pkg-config

mirage-xen: depend on conf-pkg-config