Include all documentation dependencies in releases.

This adds 2 files to the Makefile SOURCES, without which I'm unable to build documentation from the latest release tarball:

xmlto -m docbook-extra.xml man repomapper.xml
warning: failed to load external entity "/tmp/guix-build-reposurgeon-3.43.drv-0/reposurgeon-3.43/docbook-extra.xml"
compilation error: file /tmp/guix-build-reposurgeon-3.43.drv-0/xmlto-xsl.xTuhr0 line 6 element include
make: *** [Makefile:47: repotool.1] Error 5

I'm a bit suspicious about all this, since 3.43 has been out for more than a month and I'm seemingly the first one to report this problem, but here we are.