ui/curses: Make control_characters[] array const

As we only use this array as input, make it const.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

ui/curses.c

@@ -529,7 +529,7 @@ static void font_setup(void)
      * Control characters are normally non-printable, but VGA does have
      * well-known glyphs for them.
      */
-    static uint16_t control_characters[0x20] = {
+    static const uint16_t control_characters[0x20] = {
       0x0020,
       0x263a,
       0x263b,