Use proper libc stubs instead of direct syscalls on OpenBSD

Make #include fcntl.h unconditional to avoid implicit definitions.

Index: source/libraries/mimalloc/src/prim/unix/prim.c
--- source/libraries/mimalloc/src/prim/unix/prim.c.orig
+++ source/libraries/mimalloc/src/prim/unix/prim.c
@@ -57,7 +57,7 @@ terms of the MIT license. A copy of the license can be
   #include <sys/sysctl.h>
 #endif
 
-#if defined(__linux__) || defined(__FreeBSD__)
+#if defined(__linux__) || defined(__FreeBSD__) && !defined(__OpenBSD__)
   #define MI_HAS_SYSCALL_H
   #include <sys/syscall.h>
 #endif
