Add some minimal handling of category sources other than static kernel
sources. We don't actually look up dynamic sources yet (that would
require extended trace records to add the file names to the trace file
since we can't assume the trace file is running on a kernel with the
same numbers.)

Make the decision to append a "src/" prefix to each file name
dependant on the category source.

Effort: CHERI upstreaming
Sponsored by: Innovate UK