Allow Public-Domain as a valid license

Closed Mike Crute requested to merge mcrute/abuild:public-domain into master

The license Public-Domain is not a valid SPDX license identifier and causes abuild sanitycheck warnings. The Alpine project does consider public domain a valid license and uses it in 34 different packages. This change adds the Public-Domain license as an override for sanitycheck to avoid warnings for these packages.

See: aports!8863 (merged)

Related: aports!8899 (merged)

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