package/octave: update _SITE to use BR2_GNU_MIRROR

The octave 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.octave.org/download
[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>

package/octave/octave.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 OCTAVE_VERSION = 9.2.0
-OCTAVE_SITE = https://ftp.gnu.org/gnu/octave
+OCTAVE_SITE = $(BR2_GNU_MIRROR)/octave
 OCTAVE_SOURCE = octave-$(OCTAVE_VERSION).tar.lz
 OCTAVE_LICENSE = GPL-3.0+
 OCTAVE_LICENSE_FILES = COPYING