Fixed compile errors due to misnamed buffersize variable

After facing compille issues on Ubuntu 16.04 LTS about undefined variable, I found the following in line 21 in `pcap.c`: > snprintf(filter_exp, PCAP_ERRBUF_SIZE, "udp port %d", port); suggesting perhaps that the declaration should be `PCAP_ERRBUF_SIZE` instead of `PCAP_BUF_SIZE`. After changing this, the compilation worked and the program runs without issues so far.