Add zlibpng.cmd to the build script

Author: Joe Drago

scripts/build.sh

@@ -35,6 +35,7 @@ cd ext/avif/ext
 $SHELL ./aom.cmd
 $SHELL ./dav1d.cmd
 $SHELL ./rav1e.cmd
+$SHELL ./zlibpng.cmd
 cd ../../..
 mkdir build
 cd build