scripts/clean-includes: Don't claim duplicate headers found when not

scripts/clean-includes: Don't claim duplicate headers found when not

When running with --check-dup-head, the script always claims it "Found
duplicate header file includes."  Fix to do it only when it actually
found some.

Fixes: d66253e46ae2 ("scripts/clean-includes: added duplicate #include check")
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20230202133830.2152150-3-armbru@redhat.com>