Guard a bit of DTLS code.

[anhu]

Repro steps:

./configure --enable-sessionexport
make all

Fixes:

src/internal.c: In function 'ImportOptions':
src/internal.c:1561:12: error: 'Options' has no member named 'dtlsStateful'
 1561 |     options->dtlsStateful = 1;
      |            ^~
  CC       examples/client/client-client.o

[SparkiDev]

retest this please

[anhu]

This failed:

./vm-run.sh dpp_config_dpp_override_prime256v1 dpp_proto_stop_at_pkex_exchange_resp dpp_qr_code_auth_mutual_p_384 2025-12-23 20:03:03,616 INFO Failure sequence: dpp_config_dpp_override_prime256v1 dpp_proto_stop_at_pkex_exchange_resp dpp_qr_code_auth_mutual_p_384

[anhu]

jenkins retest this please

Found unhandled org.jenkinsci.plugins.workflow.support.steps.AgentOfflineException exception:
Unable to create live FilePath for wolf-linux-cloud-node-sgdp0m; wolf-linux-cloud-node-sgdp0m was marked offline: Connection was broken