dtls_prng.c: Rely on automatic include of dtls_config.h As Contiki and RIOT do not generate dtls_config.h it should not be included directly but through tinydtls.h only. Change-Id: I684f60b7685fc8eae3e81d73bcdade6a28522a28 Signed-off-by: Olaf Bergmann <bergmann@tzi.org>