patch 7.4.1859

Problem:    Cannot use a function reference for "exit_cb".
Solution:   Use get_callback(). (Yegappan Lakshmanan)

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1859,
 /**/
     1858,
 /**/