patch 8.1.2225: the "last used" info of a buffer is under used

Problem:    The "last used" info of a buffer is under used.
Solution:   Add "lastused" to getbufinfo(). List buffers sorted by last-used
            field. (Andi Massimino, closes #4722)

src/evalbuffer.c

@@ -595,6 +595,10 @@ get_buffer_info(buf_T *buf)
     }
 #endif
 
+#ifdef FEAT_VIMINFO
+    dict_add_number(dict, "lastused", buf->b_last_used);
+#endif
+
     return dict;
 }