V4L/DVB (5406): Add comment why the symbols are exported.

It is not immediately obvious why the ivtv symbols are exported
in ivtv-driver.c since both ivtv-fb and the IR-blaster module
are still out-of-tree, currently being ported to be in kernel.

Added a comment so people are aware of these issues.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>