media: cec: meson: always include meson sub-directory in Makefile

'meson' directory contains two separate drivers, so it should be added
to Makefile compilation hierarchy unconditionally, because otherwise the
meson-ao-cec-g12a won't be compiled if meson-ao-cec is not selected.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 4be5e864 ("media: move CEC platform drivers to a separate directory")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>