systemd: fix build depends for patched sources

Patches routinely modify the man page source XML, invalidating the
prebuilt man pages provided by release tarballs. Patch the live ebuild
too and require the docbook XML DTDs unconditionally like we did for 215
all the time going forward so this issue isn't forgotten again during
the next version bump.
Merge pull request #994 from marineam/systemd-xml

systemd: fix build depends for patched sources