session.h: Move inclusion of string.h into .c files

string.h is required for memcmp() that is used in session.c and
dtls_debug.c. Therefore, the header file needs to be included
only in these two files.

Change-Id: I11db1c37a7cc13d8fa2826f6d3e38d3765f4a4ec

dtls_debug.c

@@ -28,6 +28,8 @@
 #include <arpa/inet.h>
 #endif
 
+#include <string.h>
+
 #ifdef HAVE_TIME_H
 #include <time.h>
 #endif

session.c

@@ -14,6 +14,8 @@
  *
  *******************************************************************************/
 
+#include <string.h>
+
 #include "session.h"
 
 #ifdef HAVE_ASSERT_H

session.h

@@ -17,8 +17,6 @@
 #ifndef _DTLS_SESSION_H_
 #define _DTLS_SESSION_H_
 
-#include <string.h>
-
 #include "tinydtls.h"
 #include "global.h"