fix xmake version

packages/z/zlib/xmake.lua

@@ -23,7 +23,7 @@ package("zlib")
     end
 
     on_fetch(function (package, opt)
-        if xmake:version():lt("2.8.7") then return end -- disable system find if the bug is present
+        if xmake.version():lt("2.8.7") then return end -- disable system find if the bug is present
         if opt.system then
             if not package:is_plat("windows", "mingw") then
                 return package:find_package("system::z", {includes = "zlib.h"})