[LyX/master] Allow compiling with Qt6

[Pavel Sanda]

On Mon, Mar 15, 2021 at 05:57:38PM +0100, Kornel Benko wrote:
> Am Mon, 15 Mar 2021 17:33:49 +0100 (CET)
> schrieb Enrico Forestieri <forenr at lyx.org>:
> 
> > commit 635a7d77ddc94c63d52494dc4d68a930faccf45f
> > Author: Enrico Forestieri <forenr at lyx.org>
> > Date:   Mon Mar 15 17:09:09 2021 +0100
> > 
> >     Allow compiling with Qt6
> 
> I had the impression we wanted to postpone compilation with Qt6 to after 2.4?

If the transition is that simple as this patch I think we should accept it now.
I am pretty sure ppl will try to adapt 2.4 to qt6 later on, so this is more safer anyway.

Pavel