CMake: Fix build when HAVE_DNS_SD and no network backend
CMake: Fix build when HAVE_DNS_SD and no network backend

When HAVE_DNS_SD is set to ON in the config, and then
WITH_NETWORK_BACKEND is set to OFF, the library wouldn't build.

Signed-off-by: Paul Cercueil <a027184a55211cd23e3f3094f1fdc728df5e0500@crapouillou.net>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio