Add header file for compcert compatibility

[gjurgensen]

Currently, compiling basis_ffi.c with compcert (the newest release, version 3.4) results in the following error "use of undeclared identifier 'S_IRUSR'". This macro, along with the other mode arguments for open(), are defined in sys/stat.h. Including this header resolves the issue.

I'm not sure why this isn't a problem when compiling under gcc. I assume one of the other header files includes sys/stat.h, but I was also under the assumption that both compcert and gcc would use the same system-provided header files. Regardless, the linux man page for open() seems to suggest including sys/stat.h (as well as sys/types.h, although this was not necessary on my machine), so including these may be wise for portability reasons.

[xrchz]

Happy to merge this upon confirmation (from anyone) that the new file also works with gcc and/or clang on GNU/Linux and macOS

[gjurgensen]

I can confirm it works on GNU/Linux (specifically Ubuntu 18.04.1). Using gcc and then clang, I generated assembly for the original basis_ffi.c, and my modified version. I compared them with diff, and the only difference was the name of the file.

Unfortunately, I don't have the means to test on macOS.