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/misc1.c b/src/misc1.c
index 0277c9e..4884f6c 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -3082,7 +3082,7 @@
     return new_fname;
 }
 
-    void
+    static void
 prepare_to_exit(void)
 {
 #if defined(SIGHUP) && defined(SIG_IGN)