cmake: add -Wno-unused-function to dns_sd_windows.c
cmake: add -Wno-unused-function to dns_sd_windows.c

We use an external 'mdns.h' in the Windows to do dns-sd for Windows.
We don't use all the functions on purpose, so don't warn when we use that file.

Set -Werror on Travis and AppVeyor to catch issues on mingw

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

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