cmake: Add zlib dependency explicitly for when building libpng statically.
cmake: Add zlib dependency explicitly for when building libpng statically.

janjachnik/Pangolin,tschmidt23/Pangolin,randi120/Pangolin,randi120/Pangolin,pinglin/Pangolin,stevenlovegrove/Pangolin,renzodenardi/Pangolin,pinglin/Pangolin,visigoth/Pangolin,stevenlovegrove/Pangolin,stevenlovegrove/Pangolin,wxdzju/Pangolin,tschmidt23/Pangolin,mp3guy/Pangolin,arpg/Pangolin,renzodenardi/Pangolin,tschmidt23/Pangolin,janjachnik/Pangolin,arpg/Pangolin,wxdzju/Pangolin,mp3guy/Pangolin,mp3guy/Pangolin,visigoth/Pangolin