win32: Add missing include for Sleep
diff --git a/src/notification_proxy.c b/src/notification_proxy.c
index 60b2e03..c5f29f5 100644
--- a/src/notification_proxy.c
+++ b/src/notification_proxy.c
@@ -32,6 +32,7 @@
 #include "common/debug.h"
 
 #ifdef WIN32
+#include <windows.h>
 #define sleep(x) Sleep(x*1000)
 #endif