Fixed zlib header issue. Fixes #15.

libpng.mod/libpng.bmx

@@ -25,6 +25,8 @@ ModuleInfo "History: Update to libpng 1.2.12"
 
 Import Pub.ZLib
 
+Import "../zlib.mod/zlib/*.h"
+
 Import "png.c"
 Import "pngerror.c"
 Import "pngget.c"