[media] smiapp: Make pixel_order_str static

pixel_order_str is only referred to in smiapp-core.c, it should be thus
static. Thanks to Hans Verkuil for pointing this out.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>