patch 8.1.1891: functions used in one file are global

Problem:    Functions used in one file are global.
Solution:   Add "static". (Yegappan Lakshmanan, closes #4840)
diff --git a/src/popupwin.c b/src/popupwin.c
index ff7aa01..a892bb6 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -28,6 +28,8 @@
     {"center", POPPOS_CENTER}
 };
 
+static void popup_adjust_position(win_T *wp);
+
 /*
  * Get option value for "key", which is "line" or "col".
  * Handles "cursor+N" and "cursor-N".
@@ -982,7 +984,7 @@
 /*
  * Adjust the position and size of the popup to fit on the screen.
  */
-    void
+    static void
 popup_adjust_position(win_T *wp)
 {
     linenr_T	lnum;