package/aumix: update AUMIX_SITE
The old URL gives 403 forbidden.
Use a working sourceforge URL.
Fixes:
http://autobuild.buildroot.org/results/c0c3945cade7a6d7a615ac23523c93b02dbb056f
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>