headers: sync drm_sarea.h with airlied/drm-next

Adds the C++ extern guards from
ebbb0e5cfd2ceb1150b1 drm: add extern C guard for the UAPI headers

Generated using `make headers_install` from airlied/drm-next
at commit 2a1720376adda5ecf8e636fbfb05339c7dad1c55

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>