Fix build issue on mac

[maskit]

QUICPacketHandler_quiche.cc:194:54: error: cannot initialize a parameter of type 'size_t *' (aka 'unsigned long *') with an rvalue of type 'uint64_t *' (aka 'unsigned long long *')
  uint8_t *buf = udp_packet->get_entire_chain_buffer(&buf_len);
                                                     ^~~~~~~~
./I_UDPPacket.h:76:44: note: passing argument to parameter 'buf_len' here
  uint8_t *get_entire_chain_buffer(size_t *buf_len);
                                           ^