Bug#1016765: webkit2gtk: Please enable gtk4 build

[Jeremy Bicha]

I was able to successfully build your branch in my Ubuntu PPA for at
least some of the architectures. I was able to use GNOME Builder 43
(latest preview build) and Epiphany 43 Beta which both require webkit
2.37.1. GNOME Builder used the new 5.0 API (gtk4) and Epiphany used
4.1 (gtk3 + libsoup3).

I think the only concern is how long this will take to build on all
our architectures. But hopefully, it will be ok.

Thank you,
Jeremy Bicha