Investigate `#pragma once`

[jeremylt]

Currently, we use header guards (though they are probably redundant in most cases) to protect our JiT files from multiple definitions during creation of the JiTTed code.

We could use #pragma once, if it works in all our JiT environments (SYCL and OCCA are probably the two biggest concerns). See discussion here: https://gitlab.com/petsc/petsc/-/merge_requests/6822