fix libtorch for old xmake

packages/l/libtorch/xmake.lua

@@ -20,7 +20,7 @@ package("libtorch")
     add_includedirs("include/torch/csrc/api/include")
 
     -- enable long paths for git submodule on windows
-    if is_host("windows") then
+    if is_host("windows") and set_policy then
         set_policy("platform.longpaths", true)
     end