dtls.h: Fixed prototype for dtls_init()

Change-Id: I293342a02395c74ac814b214837f64a7050d63af
diff --git a/dtls.h b/dtls.h
index 273315f..cdf4950 100644
--- a/dtls.h
+++ b/dtls.h
@@ -232,7 +232,7 @@
  * This function initializes the tinyDTLS memory management and must
  * be called first.
  */
-void dtls_init();
+void dtls_init(void);
 
 /** 
  * Creates a new context object. The storage allocated for the new