ffmpeg: 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/ffmpeg/ffmpeg.hash

@@ -1,2 +1,2 @@
 # Locally calculated
-sha256 9565236404d3515aab754283c687c0a001019003148bf7f708e643608c0690b8  ffmpeg-2.8.tar.bz2
+sha256 993ca7216aa0eac899a85fe1119dac12ef0655d67f932de6a32acc00188f8182  ffmpeg-2.8.tar.xz

package/ffmpeg/ffmpeg.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 FFMPEG_VERSION = 2.8
-FFMPEG_SOURCE = ffmpeg-$(FFMPEG_VERSION).tar.bz2
+FFMPEG_SOURCE = ffmpeg-$(FFMPEG_VERSION).tar.xz
 FFMPEG_SITE = http://ffmpeg.org/releases
 FFMPEG_INSTALL_STAGING = YES