fix(qt): explicit QVariant::fromValue<qlonglong> for DepthRole

int64_t is ambiguous with QVariant's overload set (int / uint /
qlonglong / qulonglong / bool / float / double). Wrap in
QVariant::fromValue<qlonglong> to disambiguate. Fixes the
build-linux-qt failure on the rebased v6.1.7 PR.

src/qt/transactiontablemodel.cpp

@@ -636,7 +636,7 @@ QVariant TransactionTableModel::data(const QModelIndex &index, int role) const
     case StatusRole:
         return rec->status.status;
     case DepthRole:
-        return rec->status.depth;
+        return QVariant::fromValue<qlonglong>(rec->status.depth);
     }
     return QVariant();
 }