Don't build mdnssd

This was enabled for Linux only, but Linux users should use Avahi,
which is present on most modern distributions, and implements mDNS
functionality.

Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>
(cherry picked from commit 272e9a37)

Change-Id: I5e17ff9b8bff6eae9af2f8281937afdb687c371f
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>