legal-info: depend on patch rather than extract

Switch legal-info generation dependency to patch rather than extract
so we can use debian/copyright from debian-maintained packages,
because the *orig* file alone doesn't contain the appropiate file.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>