apk may be missing mandatory license field

Alpine Linux describes the license field as mandatory:

Warning: It is not acceptable to package software with "unknown" license! If you can't find the license of the source code, please contact the author and ask them to specify the license.

https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package

The current apk implementation in fpm seems to omit this field when generating the .PKGINFO configuration file.