Keep SSL context const when hw accel is disabled

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/ssl_tls.c

@@ -826,6 +826,9 @@ static int ssl_populate_transform( mbedtls_ssl_transform *transform,
                                    const unsigned char randbytes[64],
                                    int minor_ver,
                                    unsigned endpoint,
+#if !defined(MBEDTLS_SSL_HW_RECORD_ACCEL)
+                                   const
+#endif
                                    mbedtls_ssl_context *ssl )
 {
     int ret = 0;