patch 8.1.0046: loading a session file fails if 'winheight' is big

Problem:    Loading a session file fails if 'winheight' is a big number.
Solution:   Set 'minwinheight' to zero at first.  Don't give an error when
            setting 'minwinheight' while 'winheight' is a big number.
            Fix using vertical splits. Fix setting 'minwinwidth'.
            (closes #2970)
diff --git a/src/proto/window.pro b/src/proto/window.pro
index b21b463..7ed8042 100644
--- a/src/proto/window.pro
+++ b/src/proto/window.pro
@@ -54,6 +54,7 @@
 void win_setwidth(int width);
 void win_setwidth_win(int width, win_T *wp);
 void win_setminheight(void);
+void win_setminwidth(void);
 void win_drag_status_line(win_T *dragwin, int offset);
 void win_drag_vsep_line(win_T *dragwin, int offset);
 void set_fraction(win_T *wp);