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 a29ccf6f
("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>