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/misc1.c b/src/misc1.c
index 45dcc32..ca5afef 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -2761,7 +2761,7 @@
 
 # if defined(FEAT_EVAL) || defined(PROTO)
 
-    void
+    static void
 get_cmd_output_as_rettv(
     typval_T	*argvars,
     typval_T	*rettv,