patch 9.1.0818: some global functions are only used in single files

Problem:  some global functions are only used in single files
Solution: refactor code slightly and make some more functions static
          (Yegappan Lakshmanan)

closes: #15951

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/typval.c b/src/typval.c
index 01ffef5..e57d898 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -1021,7 +1021,7 @@
 /*
  * Returns TRUE if "tv" is a type alias for a class
  */
-    int
+    static int
 tv_class_alias(typval_T *tv)
 {
     return tv->v_type == VAR_TYPEALIAS &&