tcpcrypt.h should #include <stdint.h> for uint_16_t

without this change, attempts to build something against libtcpcrypt
get errors/warnings like:

/usr/include/tcpcrypt/tcpcrypt.h:44:50: error: unknown type name ‘uint16_t’
 extern char *tcpcrypt_getsessid(char *remote_ip, uint16_t remote_port,
                                                  ^