tty: use PRIx64 to print dev_t

dev_t is %ul on x86_64 and %ull on arm32

tty.c: In function 'get_tty_driver':
tty.c:247:2: error: format '%lx' expects argument of type 'long unsigned int', but argument 4 has type 'dev_t' [-Werror=format=]
  snprintf(id, sizeof(id), "tty[%lx:%lx]", rdev, dev);

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>