libpng: bump to version 1.6.9

This also fixes the download URL as oudated versions have been
moved to the "older-releases" sub-folder.

Signed-off-by: Sven Neumann <neumann@teufel.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libpng/libpng.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-LIBPNG_VERSION = 1.6.8
+LIBPNG_VERSION = 1.6.9
 LIBPNG_SERIES = 16
 LIBPNG_SOURCE = libpng-$(LIBPNG_VERSION).tar.xz
 LIBPNG_SITE = http://downloads.sourceforge.net/project/libpng/libpng${LIBPNG_SERIES}/$(LIBPNG_VERSION)