message: add missing stdarg.h include #993

Fixes up gcc 4.1 (not specifically a supported compiler but it's standard
for varargs anyway so it's good to have it included and depend less on
implicit includes).

src/nvim/message.h

@@ -2,6 +2,7 @@
 #define NVIM_MESSAGE_H
 
 #include <stdbool.h>
+#include <stdarg.h>
 #include "nvim/eval_defs.h"  // for typval_T
 
 /*