Handle SRVLOC-over-TCP like other XXX-over-TCP protocols.

No need to wrap each SRVLOC PDU, which contains a SRVLOC protocol layer,
in *another* SRVLOC protocol layer.

Change-Id: I25e05d94bb9968eb4e26b63beb09fd151820d750
Reviewed-on: https://code.wireshark.org/review/4118
Reviewed-by: Guy Harris <guy@alum.mit.edu>