webkit-gtk: Update to 2.22.6

pkgsrc changes:
- Set USE_GCC_RUNTIME to depends on gcc6-libs when pkgsrc gcc is used
  (XXX: Not tested and not clear if currently mk/compiler/gcc.mk DTRT
    XXX: regarding (if not, that's probably why firefox/mozilla-common.mk
    XXX: abuses USE_PKGSRC_GCC_RUNTIME!))

Changes:
WebKitGTK+ 2.22.6
=================
- Make kinetic scrolling slow down smoothly when reaching the ends of
  pages, instead of abruptly, to better match the GTK+ behaviour.
- Fix Web inspector magnifier under Wayland.
- Fix garbled rendering of some websites (e.g. YouTube) while scrolling
  under X11.
- Fix several crashes, race conditions, and rendering issues.

(leot)