docs: Distribute subsite.xsl

docs: Distribute subsite.xsl

So, I've ran into very interesting problem lately. When doing the
following, I've encountered an error:

  libvirt.git $ make dist && tar -xJf libvirt-2.2.0.tar.xz && \
                cd libvirt-2.2.0 && ./configure && \
                rm docs/formatdomain.html && make -C docs

  make: Entering directory 'docs'
  make: *** No rule to make target 'formatdomain.html', needed by 'web'.  Stop.
  make: Leaving directory 'docs'

I had no idea what was going on, so I've nailed down the commit
that "broke it" via running git-bisect. It was this one:
7659bd9221b9dd1cdf. But that shed no more light until I realized
that the commit might actually just exposed a problem we had. And
guess what - I've nailed it down. Of course we are not
distributing subsite.xsl that's why make prints error message.
Very misleading one I must say.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>