Add missing libevent2 compat includes.

Index: osdep-openbsd.c
--- osdep-openbsd.c.orig
+++ osdep-openbsd.c
@@ -24,6 +24,7 @@
 
 #include <errno.h>
 #include <event2/event.h>
+#include <event2/event_compat.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>