patch 8.1.0630: "wincmd p" does not work after using an autocmd window

Problem:   "wincmd p" does not work after using an autocmd window.
Solution:   Store "prevwin" in aco_save_T. (Christian Brabandt, closes #3690)

src/structs.h

@@ -3252,6 +3252,7 @@ typedef struct
     int		use_aucmd_win;	/* using aucmd_win */
     win_T	*save_curwin;	/* saved curwin */
     win_T	*new_curwin;	/* new curwin */
+    win_T	*save_prevwin;	/* saved prevwin */
     bufref_T	new_curbuf;	/* new curbuf */
     char_u	*globaldir;	/* saved value of globaldir */
 } aco_save_T;