[GTK] update-webkitgtk-libs should use mesa tarball

[GTK] update-webkitgtk-libs should use mesa tarball

https://bugs.webkit.org/show_bug.cgi?id=150470

update-webkitgtk-libs downloads all build dependencies; in fixing
bug 148941 mesa was switched from ftp to git access.  Now an http
tarball is available, which is preferred over accessing via git.

Patch by Mark Salisbury <mark.salisbury@hp.com> on 2015-10-23
Reviewed by Philippe Normand

* gtk/jhbuild.modules:
Mesa access changed to http tarball.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191498 268f45cc-cd09-0410-ab3c-d52691b4dbfc