Bug#1012697: libffi: Source URL change not documented

[Bastian Germann]

Source: libffi
Version: 3.4.2-4

The documented source URL ftp://sourceware.org/pub/libffi/ is not valid for the 
current version 3.4.2.
Sourceware does not have any 3.4.x version. Please change the d/watch and 
d/copyright file to present
the right source URL, which is most probably 
https://github.com/libffi/libffi/releases