[GTK] [JHBuild] Switch sourceware.org mirror to http.

Unreviewed.

The ftp access is recently failing to work as expected, and the
bots fail to download the file.
The same mirror is available via http and seems to work better.

* gtk/jhbuild.modules:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174209 268f45cc-cd09-0410-ab3c-d52691b4dbfc