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

Author: jhuber6

openmp/runtime/src/include/omp.h.var

@@ -15,6 +15,7 @@
 #ifndef __OMP_H
 #   define __OMP_H
 
+#   include <stddef.h>
 #   include <stdlib.h>
 #   include <stdint.h>