add struct msgbuf to sys/msg.h

include/sys/msg.h

@@ -37,6 +37,13 @@ int msgget (key_t, int);
 ssize_t msgrcv (int, void *, size_t, long, int);
 int msgsnd (int, const void *, size_t, int);
 
+#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+struct msgbuf {
+	long mtype;
+	char mtext[1];
+};
+#endif
+
 #ifdef __cplusplus
 }
 #endif