dnstap does not build on macOS

[jschlyter]

Building dnstap fails with:

../dnstap/dtstream.c:1582:38: error: use of undeclared identifier 'usock'
        s.sun_len = (unsigned)sizeof(usock);

[wcawijngaards]

Fixed the issue in 4207b58 . Thanks for the report. Those system headers are just a bit different, and the compatibility code needs adjustment.
Best regards, Wouter