dns/kf5-kdnssd: Fix build error when compiled with MDNSRESPONDER option

KDE ports build with -DQT_NO_CAST_FROM_BYTEARRAY, so the implicit cast
from QByteArray to const char * is not available. Make the necessary
conversion explicit.

This is being upstreamed as well (but would only land in 5.48 or later).

PR: 228735
Submitted by: ashish
Reported by: ashish
Reviewed by: tcberner