duplicate udev trigger interest for /usr-merge

As packages move their files from / to /usr, dpkg will cease activating udev's trigger interest for hwdb.d files. In DEP17, this is documented as P2. Since there are only two packages in Debian affected by this issue, we opt for an individual mitigation and explicitly duplicate the trigger interest to both locations (M12). This is safe regardless of the other chosen mitigations as trigger activations are accumulated.