diffoscope: Upgrade 168 -> 172

In particular 170 includes rpm header fixes which stop the webpages
for rpm diffs breaking web browsers and are important in the context
of the autobuilder.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 275738c3)
Signed-off-by: Steve Sakoman <steve@sakoman.com>