package/nettle: update _SITE to use BR2_GNU_MIRROR
The nettle package is distributed on the GNU project servers. See [1]
and [2]. Buildroot has the BR2_GNU_MIRROR configuration which can
be used for that purpose. See [3].
For consistency with all other GNU packages, this commit updates
the _SITE to use BR2_GNU_MIRROR.
Note: the nettle _SITE was updated to ftp.gnu.org in commit [4].
[1] https://www.lysator.liu.se/~nisse/nettle/
[2] https://www.gnu.org/prep/ftp.html
[3] https://gitlab.com/buildroot.org/buildroot/-/blob/2024.08.2/Config.in#L286
[4] https://gitlab.com/buildroot.org/buildroot/-/commit/92f0ef5eaa74af28b2b7b7abbe021bce2aba4875
Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
(cherry picked from commit 9ccc0f5642cf173bcc8d51f778331acfea9dbde8)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Julien Olivain