Avoid warning about cpp11::perserved, e.g.

[jimhester]

C:/r_packages/pkgcheck/lib/cpp11/include/cpp11/protect.hpp:430:1: warning: 'cpp11::preserved' defined but not used [-Wunused-variable]
  430 | preserved;
      | ^~~~~~~~~

[jwood000]

@jimhester,

Any word on this?

I keep encountering this as I'm building with the docker image wch1/r-debug.

Btw, I love the package and keep up the great work!

Joseph