patch 7.4.1558

Problem:    It is not easy to find out what windows display a buffer.
Solution:   Add win_findbuf().

src/version.c

@@ -743,6 +743,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1558,
 /**/
     1557,
 /**/