pkgconf: fix upstream URL

The upstream URL has changed to
http://rabbit.dereferenced.org/~nenolod/distfiles/

, updated the
Buildroot pkgconf package accordingly.

Fixes #6638.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by:  <masrur.macece@gmail.com>
Cc: <masrur.macece@gmail.com>