zlibpng.cmd: Pass the zlib patch file to git apply

Author: wantehchang

ext/zlibpng.cmd

@@ -5,5 +5,7 @@
 : # The odd choice of comment style in this file is to try to share this script between *nix and win32.
 
 git clone -b v1.3.1 --depth 1 https://github.com/madler/zlib.git
-git apply --ignore-whitespace
+cd zlib
+git apply --ignore-whitespace ../zlib.patch
+cd ..
 git clone -b v1.6.47 --depth 1 https://github.com/glennrp/libpng.git