debian/control: Follow current policy for Priority field

Policy 2.5 explains:

 The priority of a package is determined solely by the functionality
 it provides directly to the user. The priority of a package should
 not be increased merely because another higher-priority package
 depends on it


  This priority is deprecated. Use the optional priority instead.

This means we can treat liblzma5 and xzdec as optional now.
2 files changed