qt: Fix warning about missing return

* lang/qt/src/qgpgmeexportjob.cpp (ExportJob::exec): Return a value and
remove unused argument names.
--

ExportJob::exec is never executed because its virtual overload is always
used. Therefore it doesn't matter what value we return.

lang/qt/src/qgpgmeexportjob.cpp

@@ -107,8 +107,9 @@ void ExportJob::setExportFlags(unsigned int)
 }
 
 /* For ABI compat not pure virtual. */
-GpgME::Error ExportJob::exec(const QStringList &patterns, QByteArray &data)
+GpgME::Error ExportJob::exec(const QStringList &, QByteArray &)
 {
+    return Error();
 }
 
 #include "qgpgmeexportjob.moc"