Implement support for MBEDTLS_PSA_CRYPTO_BUILTIN_KEYS

According to the design in psa-driver-interface.md. Compiles without
issue in test_psa_crypto_drivers.

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>
diff --git a/library/psa_crypto_driver_wrappers.h b/library/psa_crypto_driver_wrappers.h
index e499411..e82d093 100644
--- a/library/psa_crypto_driver_wrappers.h
+++ b/library/psa_crypto_driver_wrappers.h
@@ -68,6 +68,11 @@
     const psa_key_attributes_t *attributes,
     uint8_t *key_buffer, size_t key_buffer_size, size_t *key_buffer_length );
 
+psa_status_t psa_driver_wrapper_get_builtin_key(
+    psa_drv_slot_number_t slot_number,
+    psa_key_attributes_t *attributes,
+    uint8_t *key_buffer, size_t key_buffer_size, size_t *key_buffer_length );
+
 /*
  * Cipher functions
  */