gnu: epiphany: Use webkitgtk-2.22.

* gnu/packages/gnome.scm (epiphany)[native-inputs]: Add gcc-7.
[inputs]: Replace webkitgtk with webkitgtk-2.22.

gnu/packages/gnome.scm

@@ -4187,6 +4187,7 @@ (define-public epiphany
      `(("dconf" ,dconf)))
     (native-inputs
      `(("desktop-file-utils" ,desktop-file-utils) ; for update-desktop-database
+       ("gcc" ,gcc-7)  ; needed because webkitgtk-2.22 is compiled with gcc-7
        ("glib:bin" ,glib "bin") ; for glib-mkenums
        ("gtk+:bin" ,gtk+ "bin") ; for gtk-update-icon-cache
        ("intltool" ,intltool)
@@ -4207,7 +4208,7 @@ (define-public epiphany
        ("libxslt" ,libxslt)
        ("nettle" ,nettle) ; for hogweed
        ("sqlite" ,sqlite)
-       ("webkitgtk" ,webkitgtk)))
+       ("webkitgtk" ,webkitgtk-2.22)))
     (home-page "https://wiki.gnome.org/Apps/Web")
     (synopsis "GNOME web browser")
     (description