[OpenMP] Add an 'stddef.h' include to 'omp.h' (#73876)

Summary:
We use `size_t` internally in the omp.h header, which is normally
provided by `stdlib.h` which is already included. Howevever, some cases
when using `-ffreestanding` can result in this not being defined via
`stdlib.h`. This patch simply adds an explicit inclusion of this header,
which is provided by the `clang` resource directory, to resolve this in
all cases.