media: media/radio/Kconfig: add back RADIO_ISA

Patch 258c524b inadvertently removed the 'select RADIO_ISA' line for
the RADIO_RTRACK.

Fixes: 258c524b ("radio: allow building ISA drivers with COMPILE_TEST")

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>