Fix build for io_uring (facebook#9690)

Summary:
Minor fix for build failure:
```
./env/io_posix.h:68:33: error: unused parameter 'len' [-Werror=unused-parameter]
   68 |                          size_t len, size_t iov_len, bool async_read,
      |                          ~~~~~~~^~~
```
Only happens for release build with io_uring.

Pull Request resolved: facebook#9690

Test Plan: build pass with io_uring

Reviewed By: akankshamahajan15

Differential Revision: D34846347

Pulled By: jay-zhuang

fbshipit-source-id: 2d7afb585097262d7722ef1beac486fc8ef28419

env/io_posix.h

@@ -97,6 +97,9 @@ inline void UpdateResult(struct io_uring_cqe* cqe, const std::string& file_name,
                             cqe->res);
     }
   }
+#ifdef NDEBUG
+  (void)len;
+#endif
 }
 #endif