buildsystem: adjust uninstalled -client/server pkg-config infra

Add missing protocol dir to uninstalled include path.

Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>