coap_gnutls.c: enable cipher suites required by RFC 7252
coap_gnutls.c: enable cipher suites required by RFC 7252

As discussed in issue #286 [1], the mandatory-to-implement
cipher suites TLS_PSK_WITH_AES_128_CCM_8 and TLS_ECDHE_ECDSA_AES_128_CCM_8
should be enabled if possible.

[1] https://github.com/obgm/libcoap/issues/286

Signed-off-by: Olaf Bergmann <71fe51b1f35b23ed52c04a7a1a0681625a2fdf56@tzi.org>

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap