CYGWIN: fix "defined but not used" warnings on mimalloc

Compiling under CYGWIN prints these warning messages:

In file included from ../cpython/Objects/mimalloc/prim/prim.c:22,
                 from ../cpython/Objects/mimalloc/static.c:37,
                 from ../cpython/Objects/obmalloc.c:23:
../cpython/Objects/mimalloc/prim/unix/prim.c:91:12: warning: ‘mi_prim_access’ defined but not used [-Wunused-function]
   91 | static int mi_prim_access(const char *fpath, int mode) {
      |            ^~~~~~~~~~~~~~
../cpython/Objects/mimalloc/prim/unix/prim.c:88:12: warning: ‘mi_prim_close’ defined but not used [-Wunused-function]
   88 | static int mi_prim_close(int fd) {
      |            ^~~~~~~~~~~~~
../cpython/Objects/mimalloc/prim/unix/prim.c:85:16: warning: ‘mi_prim_read’ defined but not used [-Wunused-function]
   85 | static ssize_t mi_prim_read(int fd, void* buf, size_t bufsize) {
      |                ^~~~~~~~~~~~
../cpython/Objects/mimalloc/prim/unix/prim.c:82:12: warning: ‘mi_prim_open’ defined but not used [-Wunused-function]
   82 | static int mi_prim_open(const char* fpath, int open_flags) {
      |            ^~~~~~~~~~~~

Looking into the code of Objects/mimalloc/prim/unix/prim.c, there is already a conditional block to avoid the generation of such warnings.
Unfortunately, CYGWIN is missing in this list.
However, adding it is very easy and the change is self-explanatory, as you can see.

Author: carlo-bramini

Objects/mimalloc/prim/unix/prim.c

@@ -77,7 +77,7 @@ static int mi_prim_access(const char *fpath, int mode) {
   return syscall(SYS_access,fpath,mode);
 }
 
-#elif !defined(__APPLE__) && !defined(_AIX) && !defined(__OpenBSD__) && !defined(__FreeBSD__) && !defined(__sun) && !defined(__NetBSD__) // avoid unused warnings
+#elif !defined(__APPLE__) && !defined(_AIX) && !defined(__CYGWIN__) && !defined(__OpenBSD__) && !defined(__FreeBSD__) && !defined(__sun) && !defined(__NetBSD__) // avoid unused warnings
 
 static int mi_prim_open(const char* fpath, int open_flags) {
   return open(fpath,open_flags);