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/proto/screen.pro b/src/proto/screen.pro
index 7a10e3b..552453a 100644
--- a/src/proto/screen.pro
+++ b/src/proto/screen.pro
@@ -52,6 +52,7 @@
 int showmode(void);
 void unshowmode(int force);
 void clearmode(void);
+void draw_tabline(void);
 void get_trans_bufname(buf_T *buf);
 int redrawing(void);
 int messaging(void);