included checks for sys/types.h and sys/stat.h on POSIX systems

Reading the identity and PSK from file in dtls-server.c example
requires function calls from these header files.

configure.in

@@ -55,7 +55,6 @@ AC_ARG_WITH(debug,
    NDEBUG=1], 
   [])
 
-
 AC_ARG_WITH(ecc,
   [AS_HELP_STRING([--without-ecc],[disable support for TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8])],
   [],
@@ -85,6 +84,7 @@ else
   AC_CHECK_HEADERS([assert.h arpa/inet.h fcntl.h inttypes.h netdb.h netinet/in.h stddef.h stdint.h stdlib.h string.h strings.h sys/param.h sys/socket.h unistd.h])
 
   AC_CHECK_HEADERS([sys/time.h time.h])
+  AC_CHECK_HEADERS([sys/types.h sys/stat.h])
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_C_INLINE