xcp-sm: Add dependency on xen-runtime

xen-runtime provides /usr/sbin/xen-ringwatch, which is called by
the ringwatch cron job which this package installs.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #341 from euanh/build-improvements

xcp-sm: Add dependency on xen-runtime