[core] added inline function to get link from msg;

include/sxmp/sxmp.h

@@ -161,6 +161,16 @@ typedef struct __sxmsg_t {
 #define sxmsg_waitlock(m)  pthread_mutex_lock(&((m)->wait))
 #define sxmsg_waitunlock(m)  pthread_mutex_unlock(&((m)->wait))
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+  static inline sxlink_t *sxmsg_link(sxmsg_t *msg) {
+    return msg->pch->link;
+  }
+#ifdef __cplusplus
+}
+#endif
+
 typedef struct __link_rpc_entry_type {
   char *name;
   int (*rpcf)(void *, sexp_t *);