Add missing libevent2 includes.

Index: tmate.h
--- tmate.h.orig
+++ tmate.h
@@ -5,6 +5,10 @@
 #include <msgpack.h>
 #include <libssh/libssh.h>
 #include <libssh/callbacks.h>
+
+#include <event2/buffer.h>
+#include <event2/dns.h>
+#include <event2/util.h>
 #include <event2/event.h>
 
 #include "tmux.h"