psa_key_attributes_t: move slot_number to core structure

Move the `slot_number` field of `psa_key_attributes_t` to
`psa_core_key_attributes_t`. This makes ``psa_core_key_attributes_t` core`
the sole field of `psa_key_attributes_t`. This paves the way to unifying
the two structures.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index ac21e3e..388e829 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -130,7 +130,7 @@
     psa_key_slot_number_t slot_number)
 {
     attributes->MBEDTLS_PRIVATE(core).MBEDTLS_PRIVATE(flags) |= MBEDTLS_PSA_KA_FLAG_HAS_SLOT_NUMBER;
-    attributes->MBEDTLS_PRIVATE(slot_number) = slot_number;
+    attributes->MBEDTLS_PRIVATE(core).MBEDTLS_PRIVATE(slot_number) = slot_number;
 }
 
 /** Remove the slot number attribute from a key attribute structure.