Fix compilation error

Problem was:
.../akonadi/src/private/protocol.cpp:546:36: error: use of overloaded operator '<<' is ambiguous (with operand types 'QTextStream' and 'const Akonadi::Protocol::ItemFetchScope::AncestorDepth')
           << "\"AncestorDepth\":" << mAncestorDepth << ","
           ~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~

CCMAIL: sknauss@kde.org