MINOR: contrib: make the peers wireshark dissector a plugin

MINOR: contrib: make the peers wireshark dissector a plugin

The wireshark dissector could only be build within wireshark, which
means maintaining a wireshark binary just for this dissector. It was not
really convenient to update wireshark because of this.

This patch converts the dissector into a .so plugin which is built with
the .h found in distributions instead of the whole wireshark sources.

(cherry picked from commit 2be58f758402010f35d67279bec7125c9665e936)
Signed-off-by: Willy Tarreau <w@1wt.eu>