patch 8.2.0514: several global functions are used in only one file

Problem:    Several global functions are used in only one file.
Solution:   Make the functions static. (Yegappan Lakshmanan, closes #5884)

src/getchar.c

@@ -99,7 +99,7 @@ static int	inchar(char_u *buf, int maxlen, long wait_time);
 /*
  * Free and clear a buffer.
  */
-    void
+    static void
 free_buff(buffheader_T *buf)
 {
     buffblock_T	*p, *np;