[media] doc-rst: add media/uapi/rc/lirc-header.rst

changeset 68cd5e0b

 ("[media] doc-rst: add LIRC header to the book")
did everything but adding the lirc-reader.rst :-p

My fault: I forgot to do a git add for this guy on such
changeset.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>