Contribs: update PNG to 1.6.12

(cherry picked from commit ce32db6caea92daebfb83d5692939798bd8a18ac)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/png/SHA512SUMS

-f475ba5544b11064544a1571e194a7e4cf387d386cd9f300f8bbdc252636a141ac2beeff47f32ee45617d894a8dee8c0c81e71ba3d0c42a47160ce32078cd1b5  libpng-1.6.10.tar.xz
+7cdf40b51a29f4df6f5be2c6946de80aa74c54169ceef2ed089fad21bf035b5672ad7e8983ca4d72edd4bb84c9d4e0077479a20e526e27685d30da03230bbb91  libpng-1.6.12.tar.xz

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.6.10
+PNG_VERSION := 1.6.12
 PNG_URL := $(SF)/libpng/libpng16/$(PNG_VERSION)/libpng-$(PNG_VERSION).tar.xz
 
 PKGS += png