libass: change tarball compression to xz

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libass/libass.mk

@@ -5,6 +5,7 @@
 ################################################################################
 
 LIBASS_VERSION = 0.10.2
+LIBASS_SOURCE = libass-$(LIBASS_VERSION).tar.xz
 LIBASS_SITE = http://libass.googlecode.com/files
 LIBASS_INSTALL_STAGING = YES
 LIBASS_LICENSE = ISC