cmake: fix custom openxr target builds
MR.2401 made a change where the state_tracker directory
is now conditionally recursed into only if the main
openxr feature is enabled, this breaks derived runtimes
which have custom openxr/serivce/service-lib targets
that depend on the oxr target.
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2422>