media: cec: add stm32 driver

Missing stm32 directory to Makefile.

Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 4be5e864 ("media: move CEC platform drivers to a separate directory")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>