Bug#1012697: libffi: Source URL change not documented

[Bastian Germann]

Control: tags -1 patch

A watch file that works for the current release and possible later GitHub 
releases is attached.
version=4
opts=uversionmangle=s/-(rc[0-9]+)$/~$1/ \
  https://github.com/libffi/libffi/releases .*/libffi-(\d\S+)\.tar.*