package/xenomai: fixup download URL

As reported in [1], https://xenomai.org/downloads/xenomai/stable has
been moved to https://ftp.denx.de/pub/xenomai/xenomai/stable.

Fixes:
- http://autobuild.buildroot.org/results/83061154a9c21ecf981fdf5b4c02be6a793a8e56

[1] https://lore.kernel.org/xenomai/95d4f7a6-1237-4ae0-9fb6-3979fa058bcc@siemens.com

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/xenomai/xenomai.mk

@@ -14,7 +14,7 @@ XENOMAI_SITE = $(call qstrip,$(BR2_PACKAGE_XENOMAI_REPOSITORY))
 XENOMAI_SITE_METHOD = git
 else
 XENOMAI_SOURCE = xenomai-$(XENOMAI_VERSION).tar.bz2
-XENOMAI_SITE = http://xenomai.org/downloads/xenomai/stable
+XENOMAI_SITE = https://ftp.denx.de/pub/xenomai/xenomai/stable
 endif
 # We're patching configure.ac
 XENOMAI_AUTORECONF = YES