Work around an old GCC bug in memory_manager.h (#529)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/529

We were hitting https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81429 "maybe_unused attribute triggers syntax error when used on first argument to a constructor" when building with older versions of GCC, like v8.5.0 on our devvms.

Rather than changing things everywhere by modifying compiler.h to avoid using `[[maybe_unused]]`, patch this one spot. It's deprecated anyway, and should go away soon.
ghstack-source-id: 202346140
exported-using-ghexport

Reviewed By: digantdesai

Differential Revision: D49759354

fbshipit-source-id: 9e912d96f4b162f456ec88daca79a6e5da33af16