contrib: png: bump to 1.6.35

contrib/src/png/SHA512SUMS

-89407c5abc1623faaa3992fc1e4a62def671d9a7401108dfceee895d5f16fe7030090bea89b34a36d377d8e6a5d40046886991f663ce075d1a2d31bf9eaf3c51  libpng-1.6.34.tar.xz
+b69152bc7869e98536effa8e45e2dac7bffee26aa9e31fc90b37506a702256919bcf4dd9cadc42fa4e45143bc023ffd7c637742367e58707e8c2226b3a09eee0  libpng-1.6.35.tar.xz

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.6.34
+PNG_VERSION := 1.6.35
 PNG_URL := $(SF)/libpng/libpng16/$(PNG_VERSION)/libpng-$(PNG_VERSION).tar.xz
 
 PKGS += png