patch 8.2.3320: some local functions are not static

Problem:    Some local functions are not static.
Solution:   Add "static".  Move snprintf() related code to strings.c.
            (Yegappan Lakshmanan, closes #8734)
diff --git a/src/getchar.c b/src/getchar.c
index 196a3ae..52c7721 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -631,7 +631,7 @@
     add_buff(&readbuf2, s, -1L);
 }
 
-    void
+    static void
 stuffReadbuffLen(char_u *s, long len)
 {
     add_buff(&readbuf1, s, len);