iw: use BR2_KERNEL_MIRROR for SITE

As pointed by Arnout Vandecappelle.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/iw/iw.mk

@@ -6,7 +6,7 @@
 
 IW_VERSION = 3.8
 IW_SOURCE = iw-$(IW_VERSION).tar.bz2
-IW_SITE = http://www.kernel.org/pub/software/network/iw
+IW_SITE = $(BR2_KERNEL_MIRROR)/software/network/iw
 IW_LICENSE = iw license
 IW_LICENSE_FILES = COPYING
 IW_DEPENDENCIES = host-pkgconf libnl