d/rules: do not compress /usr/share/doc/xen/html

d/rules: do not compress /usr/share/doc/xen/html

So, we have a xen-doc package which is already split off nicely as
binary package to contain the upstream html collection of documentation.

By default, dh_compress will gzip -9 .txt files which it considers to
be too large. However, in our case, there's some index.html with links,
and if this happens, the user who consciously installs the xen-doc
package, wanting to waste disk space on this ends up with a confusing
pile of broken links when navigating to file:///usr/share/doc/xen/html/
and browsing around.

The difference between disk space occupied when not compressing is 5.2M
vs. 5.1M before.

So, add an exclude on /usr/share/doc/xen/html

Closes: #942611
Reported-by: Diederik de Haas <didi.debian@cknow.org>
Signed-off-by: Hans van Kranenburg <hans@knorrie.org>