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)
diff --git a/src/evalbuffer.c b/src/evalbuffer.c
index 35c9ed2..a82b897 100644
--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -595,6 +595,10 @@
     }
 #endif
 
+#ifdef FEAT_VIMINFO
+    dict_add_number(dict, "lastused", buf->b_last_used);
+#endif
+
     return dict;
 }