drm/etnaviv: add extern C guard for the UAPI header

Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>