Use mmap() within code_gen_alloc() for OpenBSD.

Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/exec.c b/exec.c
index a338495..49c28b1 100644
--- a/exec.c
+++ b/exec.c
@@ -517,7 +517,8 @@
             exit(1);
         }
     }
-#elif defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || defined(__DragonFly__)
+#elif defined(__FreeBSD__) || defined(__FreeBSD_kernel__) \
+    || defined(__DragonFly__) || defined(__OpenBSD__)
     {
         int flags;
         void *addr = NULL;