Don't define field in ExternalFileHandler that's not used on Windows.

This fixes:
```
error: private field 'buffer_aligned_size_' is not used [-Werror,-Wunused-private-field]
   87 |   int64 buffer_aligned_size_{};
      |         ^
```
in the downstream Chrome build.
PiperOrigin-RevId: 565221813

mediapipe/tasks/cc/core/external_file_handler.h

@@ -82,9 +82,11 @@ class ExternalFileHandler {
 
   // The aligned mapped memory buffer offset, if any.
   int64 buffer_aligned_offset_{};
+#ifndef _WIN32
   // The aligned mapped memory buffer size in bytes taking into account the
   // offset shift introduced by buffer_aligned_memory_offset_, if any.
   int64 buffer_aligned_size_{};
+#endif
 };
 
 }  // namespace core