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>

package/nettle/nettle.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 NETTLE_VERSION = 3.10
-NETTLE_SITE = https://ftp.gnu.org/gnu/nettle
+NETTLE_SITE = $(BR2_GNU_MIRROR)/nettle
 NETTLE_DEPENDENCIES = host-m4 gmp
 NETTLE_INSTALL_STAGING = YES
 NETTLE_LICENSE = Dual GPL-2.0+/LGPL-3.0+