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/fileio.c b/src/fileio.c
index e07b908..606359a 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -111,7 +111,7 @@
 #endif
 static char *e_auchangedbuf = N_("E812: Autocommands changed buffer or buffer name");
 
-    void
+    static void
 filemess(
     buf_T	*buf,
     char_u	*name,