Provide an Appstream metadata.xml file for the USB ids serviced

The Appstream metadata file included here will prevent Lintian from complaining that libnitrokey does not properly advertise the USB ids it services via Appstream.

This file should be forwarded to upstream and be distributed by them as noted here.

The maintainer of this package noted in Debian Bug#916911 that it would make sense for the app, rather than the library, to provide the metadata. Upstream already made efforts to distribute similar but possibly incomplete information with the package nitrokey-app.

Unfortunately, that separates the appstream data from the udev rules. Perhaps the udev rules shipped in this package under data/41-nitrokey_old.rules should instead be shipped with the package nitrokey-app.

The sole purpose of this commit is to close Debian Bug#916735 against Lintian.