mimalloc compile error on OpenBSD

[colesbury]

Bug report

In file included from Objects/mimalloc/prim/prim.c:22,
                 from Objects/mimalloc/static.c:37,
                 from Objects/obmalloc.c:22:
Objects/mimalloc/prim/unix/prim.c: In function 'mi_prim_open':
Objects/mimalloc/prim/unix/prim.c:67:10: error: implicit declaration of function 'syscall'; did you mean 'sysconf'? [-Werror=implicit-function-declaration]
   return syscall(SYS_open,fpath,open_flags,0);
          ^~~~~~~
          sysconf

Reported in https://discuss.python.org/t/compiling-python3-13-alpha-release-on-openbsd/50273

Linked PRs

• gh-117547: Fix mimalloc compile error on OpenBSD #117548