tty: vt/vt: fix sparse warning

this patch fixes following sparse warning:
vt.c:1240:12: warning: symbol 'rgb_from_256' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -1237,7 +1237,7 @@ static void default_attr(struct vc_data *vc)
 
 struct rgb { u8 r; u8 g; u8 b; };
 
-struct rgb rgb_from_256(int i)
+static struct rgb rgb_from_256(int i)
 {
 	struct rgb c;
 	if (i < 8) {            /* Standard colours. */