The `psa_core_key_attributes_t` structure is no longer used. Remove it.
Switch `psa_key_attributes_t` back to a simple struct, now containing
the fields that were formerly inside its `psa_core_key_attributes_t core`
member. This repairs the build with non-C11 compilers.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>