pk: fix unused variable in copy_from_psa()

key_bits is unused when neither MBEDTLS_RSA_C or MBEDTLS_PK_HAVE_ECC_KEYS
are defined.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

library/pk.c

@@ -968,6 +968,7 @@ static int copy_from_psa(mbedtls_svc_key_id_t key_id,
     } else
 #endif /* MBEDTLS_PK_HAVE_ECC_KEYS */
     {
+        (void) key_bits;
         return MBEDTLS_ERR_PK_BAD_INPUT_DATA;
     }