Bug 29951: Fix EXPORT for C4::ClassS*Routine modules

Bug 29951: Fix EXPORT for C4::ClassS*Routine modules

Can't locate object method "subclasses" via package "C4::ClassSplitRoutine" at /kohadevbox/koha/C4/ClassSplitRoutine.pm line 53

Certainly from bug 17600.

Test plan:
Home -> Administration -> Classification sources -> New splitting rule

And create classification sources and filing rules.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>