Partial compile fix for current Qt dev and/or Qt 6

This does not cover Utils::MapReduceOption with QVector, and code
that's not compiled on my machine.

Change-Id: Ib63923985c52b1bb74e5ec2068a2bb37469ac618
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/git/gerrit/gerritmodel.cpp

@@ -55,6 +55,8 @@ enum { debug = 0 };
 
 using namespace VcsBase;
 
+using namespace Qt;
+
 namespace Gerrit {
 namespace Internal {