Add comments for framework-connectivity-t-mdns-standalone-build-sources

framework-connectivity-t-mdns-standalone-build-sources should only be
used for building service-connectivity-mdns-standalone-build-test. Added
comment to prevent this filegroup being used in wrong way.

Test: TH
Change-Id: I11dd390967db1a115dcadcdebc20f226ab6c0a51