Suppres warning about archive timestamp (#3044)

Summary:
Currently, for each third party library checked out with ExternalProject_Add, the following warning is shown.

This commit set the policy so that the warning is not shown.

```
CMake Warning (dev) at ci_env/lib/python3.10/site-packages/cmake/data/share/cmake-3.25/Modules/ExternalProject.cmake:3075 (message):
  The DOWNLOAD_EXTRACT_TIMESTAMP option was not given and policy CMP0135 is
  not set.  The policy's OLD behavior will be used.  When using a URL
  download, the timestamps of extracted files should preferably be that of
  the time of extraction, otherwise code that depends on the extracted
  contents might not be rebuilt if the URL changes.  The OLD behavior
  preserves the timestamps from the archive instead, but this is usually not
  what you want.  Update your project to the NEW behavior or specify the
  DOWNLOAD_EXTRACT_TIMESTAMP option with a value of true to avoid this
  robustness issue.
```

Pull Request resolved: https://github.com/pytorch/audio/pull/3044

Reviewed By: xiaohui-zhang

Differential Revision: D43110818

Pulled By: mthrok

fbshipit-source-id: d2e20c9fdbbeeedb5ad546fe32dbda28c5bdd431

CMakeLists.txt

@@ -16,6 +16,9 @@ if(NOT CMAKE_VERSION VERSION_LESS 3.15.0)
   cmake_policy(SET CMP0092 NEW)
 endif()
 
+# Suppress warning about ExternalProject_Add timestamp
+cmake_policy(SET CMP0135 NEW)
+
 project(torchaudio)