[media] cec-notifier.h: handle unreachable CONFIG_CEC_CORE

Fix a link error in this specific combination of config options:

CONFIG_MEDIA_CEC_SUPPORT=y
CONFIG_CEC_CORE=m
CONFIG_MEDIA_CEC_NOTIFIER=y
CONFIG_VIDEO_STI_HDMI_CEC=m
CONFIG_DRM_STI=y

drivers/gpu/drm/sti/sti_hdmi.o: In function `sti_hdmi_remove':
sti_hdmi.c:(.text.sti_hdmi_remove+0x10): undefined reference to
`cec_notifier_set_phys_addr'
sti_hdmi.c:(.text.sti_hdmi_remove+0x34): undefined reference to
`cec_notifier_put'
drivers/gpu/drm/sti/sti_hdmi.o: In function `sti_hdmi_connector_get_modes':
sti_hdmi.c:(.text.sti_hdmi_connector_get_modes+0x4a): undefined
reference to `cec_notifier_set_phys_addr_from_edid'
drivers/gpu/drm/sti/sti_hdmi.o: In function `sti_hdmi_probe':
sti_hdmi.c:(.text.sti_hdmi_probe+0x204): undefined reference to
`cec_notifier_get'
drivers/gpu/drm/sti/sti_hdmi.o: In function `sti_hdmi_connector_detect':
sti_hdmi.c:(.text.sti_hdmi_connector_detect+0x36): undefined reference
to `cec_notifier_set_phys_addr'
drivers/gpu/drm/sti/sti_hdmi.o: In function `sti_hdmi_disable':
sti_hdmi.c:(.text.sti_hdmi_disable+0xc0): undefined reference to
`cec_notifier_set_phys_addr'

The version below seems to work, though I don't particularly
like the IS_REACHABLE() addition since that can be confusing
to users.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>