dh_setup_copyright: Fix build failure if dwarf2sources fails

dwarf2sources failures are supposed to be ignored (hence the use of doit_noerror when invoking the command), but the recently added steps to extract copyright information with fail the build if the dwarf2sources result file is not found. In that case, just skip extraction altogether.

https://phabricator.apertis.org/T8985

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com