mktables: Mention USourceData in generated pod

mktables: Mention USourceData in generated pod

These files were included by Unicode for the first time in the final
version of its version 6.2.  They document proposals for encoding
Han characters in Unicode.  As far as I can tell, they have no real use
except to people working on such proposals.  They are considered part of
the Unicode Character Database, however, and should be mentioned in
perluniprops as data that Perl ignores from that database.