Fix 2 warnings found with Clang 3.5

dnspacket.cc:599:105: fatal error: taking the absolute value of unsigned type 'unsigned long' has no effect [-Wabsolute-value]
    L<<Logger::Error<<"Packet for '"<<q->qdomain<<"' denied: TSIG (key '"<<*keyname<<"') time delta "<< abs(trc->d_time - now)<<" > 'fudge' "<<trc->d_fudge<<endl;
                                                                                                        ^
dnspacket.cc:599:105: note: remove the call to 'abs' since unsigned values cannot be negative
    L<<Logger::Error<<"Packet for '"<<q->qdomain<<"' denied: TSIG (key '"<<*keyname<<"') time delta "<< abs(trc->d_time - now)<<" > 'fudge' "<<trc->d_fudge<<endl;
                                                                                                        ^~~