patch 8.1.2021: some global functions can be local to the file

Problem:    Some global functions can be local to the file.
Solution:   Add "static". (Yegappan Lakshmanan, closes #4917)
diff --git a/src/filepath.c b/src/filepath.c
index 811682e..5ffb8c9 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -793,7 +793,7 @@
     rettv->vval.v_number = filewritable(tv_get_string(&argvars[0]));
 }
 
-    void
+    static void
 findfilendir(
     typval_T	*argvars UNUSED,
     typval_T	*rettv,