package/pkgconf: fix download URL

Due to problems with the registrar (dynadot) [1], the dereferenced.org
domain got stolen and is up for auction. While sorting that out, the
official download site has changed to a different domain [2]. Update
PKGCONF_SITE to the new location.

There's an upstream bug to track the issue [3].

[1] https://social.treehouse.systems/@ariadne/110643909699308207
[2] https://github.com/pkgconf/pkgconf/commit/437c2a3218bfcb1cae7fa38a4ccd0cb29575ff07
[3] https://github.com/pkgconf/pkgconf/issues/302

Fixes: https://bugs.busybox.net/show_bug.cgi?id=15673

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>