sockets: tls: Enable access to mbedtls private fields

Several fields of structures in mbedTLS 3.0 are now private. To access
them directly is necessary to define MBEDTLS_ALLOW_PRIVATE_ACCESS.

That is a temporary fix, the proper solution is not access directly
but using proper API.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>