fconfig: change upstream site

The upstream site is dead, used http://sources.buildroot.net
temporarly.

Fixes:

  http://autobuild.buildroot.org/results/778/77899f882e04121ca5f2b96f0df9f24ead26d12b/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>