Fixed warning for snprintf call

This change seems to fix the following compilation warning:
alert.cpp: In member function 'virtual std::string libtorrent::dht_reply_data_alert::message() const':
alert.cpp:357:86: warning: format '%d' expects argument of type 'int', but argument 4 has type 'std::list<libtorrent::entry>::size_type {aka long unsigned int}' [-Wformat=]
   snprintf(msg, sizeof(msg), "reply to dht getData received %d entries", m_lst.size());
                                                                                      ^
alert.cpp:357:86: warning: format '%d' expects argument of type 'int', but argument 4 has type 'std::list<libtorrent::entry>::size_type {aka long unsigned int}' [-Wformat=]