patch 8.1.0706: tabline is not always redrawn

Problem:    Tabline is not always redrawn when something that is used in
            'tabline' changes.
Solution:   Add ":redrawtabline" so that a plugin can at least cause the
            redraw when needed.
diff --git a/src/screen.c b/src/screen.c
index e9405b4..04f19d7 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -154,7 +154,6 @@
 static void win_rest_invalid(win_T *wp);
 static void msg_pos_mode(void);
 static void recording_mode(int attr);
-static void draw_tabline(void);
 static int fillchar_status(int *attr, win_T *wp);
 static int fillchar_vsep(int *attr);
 #ifdef FEAT_MENU
@@ -10693,7 +10692,7 @@
 /*
  * Draw the tab pages line at the top of the Vim window.
  */
-    static void
+    void
 draw_tabline(void)
 {
     int		tabcount = 0;