(manpages): Fix inclusion of `defs.man'.

(manpages): Fix inclusion of `defs.man'.

When the manpage machinery was last turned upside-down, way back in
2008 (commit e99aedcf9373b3305c32e510c086bf3357b4736a), all of the
manpages were fixed to say `.so ../common/defs.man' instead of `.so
../defs.man.in'.  Inexplicably, all but one of the manpages added since
then have had the wrong include request.  Fix this silliness.

Note that this only affects formatting the manpages directly from the
source tree: there's build machinery which embeds the common definitions
in the built manpages which are installed or packaged, and that works
just fine.  But it's still good to get this stuff right.

Just maybe I can avoid introducing more broken manpages in the future.