Add missing libevent2 compat includes.

Index: tmux.h
--- tmux.h.orig
+++ tmux.h
@@ -26,7 +26,14 @@
 #include <sys/time.h>
 #include <sys/uio.h>
 
+#include <event2/buffer.h>
+#include <event2/buffer_compat.h>
+#include <event2/bufferevent.h>
+#include <event2/bufferevent_compat.h>
+#include <event2/bufferevent_struct.h>
+#include <event2/event_struct.h>
 #include <event2/event.h>
+#include <event2/event_compat.h>
 #include <limits.h>
 #include <stdarg.h>
 #include <stdio.h>