[media] lirc: fix non-ANSI function declaration warning

Sparse is warning about non-ANSI function declaration.
Add void to the parameterless function.

drivers/staging/media/lirc/lirc_bt829.c:174:22: warning:
	non-ANSI function declaration of function 'poll_main'

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>