linux-headers: switch to xz format

The headers and kernels where changed to XZ format on commit
98b5cc3eb4d4103b2149325b98a5e93e68c68dd8, but the headers reverted back
to bz2 on the packaging of the toolchain.
This causes double kernel downloads when the versions match, so switch
back the headers to XZ.

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

package/linux-headers/linux-headers.mk

@@ -13,7 +13,7 @@ LINUX_HEADERS_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v2.6/
 else
 LINUX_HEADERS_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v3.x/
 endif
-LINUX_HEADERS_SOURCE = linux-$(LINUX_HEADERS_VERSION).tar.bz2
+LINUX_HEADERS_SOURCE = linux-$(LINUX_HEADERS_VERSION).tar.xz
 
 LINUX_HEADERS_INSTALL_STAGING = YES