package/freeipmi: update _SITE to use BR2_GNU_MIRROR

The freeipmi 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.

[1] https://www.gnu.org/software/freeipmi/download.html
[2] https://www.gnu.org/prep/ftp.html
[3] https://gitlab.com/buildroot.org/buildroot/-/blob/2024.08.2/Config.in#L286

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
(cherry picked from commit bfa2dbc2d679b88d8166359ec916590050fb5802)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/freeipmi/freeipmi.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 FREEIPMI_VERSION = 1.6.14
-FREEIPMI_SITE = https://ftp.gnu.org/gnu/freeipmi
+FREEIPMI_SITE = $(BR2_GNU_MIRROR)/freeipmi
 FREEIPMI_LICENSE = GPL-3.0+, BSD-like (sunbmc)
 FREEIPMI_LICENSE_FILES = \
 	COPYING COPYING.bmc-watchdog COPYING.ipmiconsole COPYING.ipmi-dcmi \