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>

dtls_prng.c

diff --git a/dtls_prng.c b/dtls_prng.c
index 2fc3ff7..0726de8 100644
--- a/dtls_prng.c
+++ b/dtls_prng.c
@@ -16,7 +16,6 @@
  *
  *******************************************************************************/
 
-#include "dtls_config.h"
 #include "tinydtls.h"
 
 #if defined (WITH_CONTIKI)