patch 9.0.1554: code for handling 'switchbuf' is repeated

Problem:    Code for handling 'switchbuf' is repeated.
Solution:   Add a function to handle 'switchbuf'. (Yegappan Lakshmanan,
            closes #12397)
diff --git a/src/proto/window.pro b/src/proto/window.pro
index d1108af..72612b8 100644
--- a/src/proto/window.pro
+++ b/src/proto/window.pro
@@ -1,6 +1,7 @@
 /* window.c */
 int window_layout_locked(enum CMD_index cmd);
 win_T *prevwin_curwin(void);
+win_T *swbuf_goto_win_with_buf(buf_T *buf);
 void do_window(int nchar, long Prenum, int xchar);
 void get_wincmd_addr_type(char_u *arg, exarg_T *eap);
 int win_split(int size, int flags);