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/viminfo.c

@@ -2152,7 +2152,7 @@ write_viminfo_filemarks(FILE *fp)
 /*
  * Compare functions for qsort() below, that compares b_last_used.
  */
-    static int
+    int
 buf_compare(const void *s1, const void *s2)
 {
     buf_T *buf1 = *(buf_T **)s1;