V4L/DVB: ir-core: make ir_g_keycode_from_table a public function

The imon driver I've previously submitted and have been porting to
use ir-core needs to use ir_g_keycode_from_table, as ir_keydown is
not sufficient, due to these things having really oddball hardware
decoders in them. This just moves the function declaration from
ir-core-priv.h over to ir-core.h.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>