Fix otherlibs/win32unix/mmap.c
Fix otherlibs/win32unix/mmap.c

This file should also include caml/alloc.h because it uses caml_copy_string
which this header defines.

This commit is a follow-up to PR#1077.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml