l2tp: Remove duplicate included header file trace.h

Remove duplicate included header file trace.h and the following warning
reported by make includecheck:

  trace.h is included more than once

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Link: https://patch.msgid.link/20240703061147.691973-2-thorsten.blum@toblux.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>