[LyX/master] Allow compiling with Qt6

[Jean-Marc Lasgouttes]

Le 16/03/2021 à 10:48, Enrico Forestieri a écrit :
>> LyX 2.4 will still support Qt 4.8. I trust this is old enough :)
> 
> So, I infer that the attached patch is needed.

Needed, I do not know. I think it still works with Qt4.5 and we had one 
user at some time, but nobody knows whether it still compiles. In any 
case, it would be easy to fix if we got a report of broken LyX with 4.5 IMO.

JMarc