perf record: Cache build-ID of hit DSOs only

It post-processes samples to find which DSO has samples.  Based on that
info, it can save used DSOs in the build-ID cache directory.  But for
some reason, it saves all DSOs without checking the hit mark.  Skipping
unused DSOs can give some speedup especially with --buildid-mmap being
default.

On my idle machine, `time perf record -a sleep 1` goes down from 3 sec
to 1.5 sec with this change.

Fixes: e29386c8 ("perf record: Add --buildid-mmap option to enable PERF_RECORD_MMAP2's build id")
Reviewed-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20250731070330.57116-1-namhyung@kernel.org


Signed-off-by: Namhyung Kim <namhyung@kernel.org>