Fix KDE_INVENT use

Not all GitLab installations are the same; KDE_INVENT is one
that still uses the "old" naming scheme, so set WRKSRC for
ports that use that (all of which as "this should be a
temporary solution until there is a real release again").
Fixes build failures (patch, actually, since the extracted
directories didn't match expectations) in e.g. audio/amarok.