zlib: bump to version 1.2.8

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/zlib/zlib.mk

@@ -4,8 +4,8 @@
 #
 ################################################################################
 
-ZLIB_VERSION = 1.2.7
-ZLIB_SOURCE =zlib-$(ZLIB_VERSION).tar.bz2
+ZLIB_VERSION = 1.2.8
+ZLIB_SOURCE = zlib-$(ZLIB_VERSION).tar.xz
 ZLIB_SITE = http://downloads.sourceforge.net/project/libpng/zlib/$(ZLIB_VERSION)
 ZLIB_LICENSE = zlib license
 ZLIB_LICENSE_FILES = README