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/autocmd.c b/src/autocmd.c
index 17b1748..640032b 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -1659,7 +1659,7 @@
 /*
  * Return TRUE when there is a CursorHold autocommand defined.
  */
-    int
+    static int
 has_cursorhold(void)
 {
     return (first_autopat[(int)(get_real_state() == NORMAL_BUSY