Process ID is not read (when available) in RFC 3164 parser

[guss77]

Example message:

  <148>Sep  5 12:53:32 vapp-172-24-41-63 asterisk[8865]: chan_sip.c:4077 in retrans_pkt: Retransmission timeout reached on transmission 14                                                              

When the message is received by go-syslog, the PID in the square brackets is not parsed - parseTag skips over the contents of the process id.