MINOR: stats: Add missing include

"proto/stats.h" must include "types/stats.h".
diff --git a/include/proto/stats.h b/include/proto/stats.h
index 02a3506..77edda9 100644
--- a/include/proto/stats.h
+++ b/include/proto/stats.h
@@ -26,6 +26,7 @@
 #include <common/config.h>
 #include <types/applet.h>
 #include <types/stream_interface.h>
+#include <types/stats.h>
 
 
 static inline enum field_format field_format(const struct field *f, int e)