Contrib: update libpng to 1.5.10

CVE-2011-3048 (serious vulnerability)
(cherry picked from commit efeb2e712313a988743307bea766edac2d748d78)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/png/SHA512SUMS

-1b8dc51ef60e5ac1f778672aeede58d9c0a5dc91a00738cfccfe06b07e80fea149d34c7961a1268f5ce3644647fdf5396ac0be623298ead4f35410ad16f68854  libpng-1.5.9.tar.bz2
+cf9394347d6ab34591fd119d375975857f95cdbc422a04e1f89f5896f29ed1d456dd0e22fd4d7428fc7effad0c143264531a632efe7295117897b45b6d563797  libpng-1.5.10.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.5.9
+PNG_VERSION := 1.5.10
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png