secure_storage: its: make definitions available outside of transform.h

Some ITS store module implementations may make use of them.
This is the case of the custom one in the
secure_storage.psa.its.secure_storage.custom.store test.

Instead of making transform.h conditionally available, move the definitions
to common.h and simply make them available whenever the ITS transform
module is enabled.

At the same time, remove unneeded/redundant includes/build asserts.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>