Contribs: update png to 1.5.17

contrib/src/png/SHA512SUMS

-5fba9227e991f7cba6b45d687bd48984d005c072f6de05cb2b6bd9cab7b816aceae85986fcf0b33ae6110dc71852f5452ef3914ed7d8a5b8f54ede3f2d8448e4  libpng-1.5.14.tar.bz2
+6f4bfd143a56c56f7ab61475b09123fe33cde95a63bc3f1f0756f708670b1a6e915c2891833e44546a18ff1254099ddf34f33d5e13cd8a61842ec53b179904e4  libpng-1.5.17.tar.bz2

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.5.14
+PNG_VERSION := 1.5.17
 PNG_URL := $(SF)/libpng/libpng-$(PNG_VERSION).tar.bz2
 
 PKGS += png