docs: Suppress "warning: macro `DD' not defined" warning

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