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>
Julien Olivain