libdevhelp: Fix .pc file requires when building with webkit2gtk-4.0

Substitute either webkit2gtk-3.0 or webkit2gtk-4.0 in the .pc file,
depending on what webkit2gtk version devhelp was built with.

https://bugzilla.gnome.org/show_bug.cgi?id=735252
release: 3.13.90
post release version bump