Contribs: update libpng to 1.5.12

(cherry picked from commit 38830c3bfd53aed07bb40f321a1490e16359c7ef)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/png/SHA512SUMS

-cf9394347d6ab34591fd119d375975857f95cdbc422a04e1f89f5896f29ed1d456dd0e22fd4d7428fc7effad0c143264531a632efe7295117897b45b6d563797  libpng-1.5.10.tar.bz2
+3edaeb2aa879438f9b71a178fb7bb52fc92589f7544de754d0fafdc5d371eaa19edfe9b771ac9cc0bb674c20108fa2d58b6aa7136f70a588d6b2ef0d1e141324  libpng-1.5.12.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.5.10
+PNG_VERSION := 1.5.12
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png