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>
1 file changed