docs: Suppress "warning: macro `DD' not defined" warning
Suppress "warning: macro `DD' not defined" warning for ovs-brcompatd.8.
As per the description by Ben Pfaff for the same problem effecting
other files:
deamon.man allows the file that is including it to include extra
text in the description of --detach by defining a macro named DD.
Only some of the manpages that included it did this (only those
manpages that needed extra text there). But it's better to be
quiet in "man --warnings", so this defines DD to an empty value in
the other manpages that include daemon.man.
Reported by lintian