drivers/video/console/promcon.c: fix build error

drivers/video/console/promcon.c:158: error: implicit declaration of
function 'con_protect_unimap'

Introduced by commit a29ccf6f82
("embedded: fix vc_translate operator precedence").

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Cc: Tim Bird <tim.bird@am.sony.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

include/linux/vt_kern.h

@@ -86,6 +86,7 @@ int con_copy_unimap(struct vc_data *dst_vc, struct vc_data *src_vc);
 #define con_copy_unimap(d, s) (0)
 #define con_get_unimap(vc, ct, uct, list) (-EINVAL)
 #define con_free_unimap(vc) do { ; } while (0)
+#define con_protect_unimap(vc, rdonly) do { ; } while (0)
 
 #define vc_translate(vc, c) (c)
 #endif