net/mDNSResponder: fix build on 9.3-RELEASE

• older lex/flex needs yylineno

PR: 210723
MFH: 2016Q3
Reported by: Ryan Frederick <ryanrfrederick@gmail.com>
Submitted by: Walter Schwarzenfeld <w.schwarzenfeld@utanet.at>
Approved by: sunpoet (maintainer timeout)