sock_dtls: make sock_dtls-header includeable without `sock_dtls` module
sock_dtls: make sock_dtls-header includeable without `sock_dtls` module

ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,jasonatran/RIOT