Add stdarg.h where missing.

diff --git a/server/event.c b/server/event.c
index c00f7d0..90c525c 100644
--- a/server/event.c
+++ b/server/event.c
@@ -24,6 +24,7 @@
 #include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdarg.h>
 
 #include "windef.h"
 #include "winternl.h"