rtp: Take the parameter from the arguments

$ gst rtp_replay.st -a FILE
1 file changed
tree: 88c0b33225223b9f7d7d0c21a8e9fe5dd57f65d6
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README