guix: fix webkitgtk variable name
guix: fix webkitgtk variable name

This change fixes this issue:
  guix environment -l guix.scm
  [...]
  /data/build-scripts/guix.scm:76:5: In procedure inputs:
  error: webkitgtk-2.26: unbound variable

With guix-1.0.1-7.fc1fe72, webkitgtk is 2.26.1.

jmercouris/NeXT,jmercouris/NeXT