patch 8.2.0514: several global functions are used in only one file

Problem:    Several global functions are used in only one file.
Solution:   Make the functions static. (Yegappan Lakshmanan, closes #5884)
diff --git a/src/drawscreen.c b/src/drawscreen.c
index 17a385c..c60d75b 100644
--- a/src/drawscreen.c
+++ b/src/drawscreen.c
@@ -73,6 +73,8 @@
 static int  did_update_one_window;
 #endif
 
+static void win_redr_status(win_T *wp, int ignore_pum);
+
 /*
  * Based on the current value of curwin->w_topline, transfer a screenfull
  * of stuff from Filemem to ScreenLines[], and update curwin->w_botline.
@@ -382,7 +384,7 @@
  * If "ignore_pum" is TRUE, also redraw statusline when the popup menu is
  * displayed.
  */
-    void
+    static void
 win_redr_status(win_T *wp, int ignore_pum UNUSED)
 {
     int		row;