build: Fix scanner path in uninstalled pkg-config file

this was generating a pkg-config file that said wayland-scanner was
wayland/src/wayland-scanner when it's actually wayland/wayland-scanner

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