contrib/credential: .gitignore libsecret build artifacts

The libsecret credential helper does not mark its build artifact as
ignored, so running "make" results in a dirty working tree.

Mark the "git-credential-libsecret" binary as ignored to avoid the above.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>