[connman] Fix dnsproxy unit test non-optimized compilation

With the default optimized compilation the compiler doesn't include
functions that aren't used into the binary, and their dependencies
aren't needed; with non-optimized compilation additional stubs are
needed to link the unit test even if those functions aren't used.
Merge pull request #135 from hmallat/unittest-compile-without-optimization

[connman] Fix dnsproxy unit test non-optimized compilation