INTERFACE libraries with SOURCES now appear in the generated
buildsystem, so include them in the codemodel output too.

We do not need to bump the `codemodel-v2` object kind minor
version because that was already done in post-3.18 development
by commit 7d6861f3 (fileapi: Extend codemodel targets with
language standard, 2020-06-18).

Fixes: #18608