vim-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 vim/vim#4722)
52410575be

src/nvim/option_defs.h

@@ -277,6 +277,7 @@ enum {
 #define WIM_FULL        1
 #define WIM_LONGEST     2
 #define WIM_LIST        4
+#define WIM_BUFLASTUSED 8
 
 // arguments for can_bs()
 #define BS_INDENT       'i'     // "Indent"