Fix unused variable in PSA core when no AEAD algorithm is present

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>

library/psa_crypto.c

@@ -4584,6 +4584,7 @@ psa_status_t psa_aead_encrypt( mbedtls_svc_key_id_t key,
     else
 #endif /* MBEDTLS_CHACHAPOLY_C */
     {
+        (void) tag;
         return( PSA_ERROR_NOT_SUPPORTED );
     }