sys: inet_pton: conditional include for atmega2560

include sys/types.h conditionally for atmega2560
    Without this change test/unittests fails to build due to a missing
    declaration of ssize_t

sys/net/crosslayer/net_help/inet_pton.c

@@ -18,6 +18,10 @@
 #include <string.h>
 #include <stdint.h>
 
+#if defined(MCU_ATMEGA2560)
+#include <sys/types.h>
+#endif
+
 #include "kernel.h"
 
 #include "inet_pton.h"