patch 9.0.1512: inserting lines when scrolling with 'smoothscroll' set

Problem:    Inserting lines when scrolling with 'smoothscroll' set.
Solution:   Adjust line height computation for w_skipcol. (Luuk van Baal,
            closes #12350)
diff --git a/src/proto/move.pro b/src/proto/move.pro
index 0858e26..1fe46a3 100644
--- a/src/proto/move.pro
+++ b/src/proto/move.pro
@@ -1,4 +1,5 @@
 /* move.c */
+int adjust_plines_for_skipcol(win_T *wp, int n);
 void redraw_for_cursorline(win_T *wp);
 void update_topline_redraw(void);
 void update_topline(void);