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/terminal.c b/src/terminal.c
index 6d9fb24..3d66bbb 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -875,7 +875,7 @@
 
 
 // Terminals that need to be freed soon.
-term_T	*terminals_to_free = NULL;
+static term_T	*terminals_to_free = NULL;
 
 /*
  * Free a terminal and everything it refers to.
@@ -6024,7 +6024,7 @@
     pResizePseudoConsole(term->tl_conpty, consize);
 }
 
-    void
+    static void
 term_free_conpty(term_T *term)
 {
     if (term->tl_siex.lpAttributeList != NULL)