add icon for mitos2

[bernt-matthias]

Was just stripping the old MITOS webserver part of the MITOS sources and found an icon.

@davelopez I tried with planemo serve and could not see the icon anywhere. http://localhost:9090/api/tools/mitos2/icon worked.

FOR CONTRIBUTOR:

• I have read the CONTRIBUTING.md document and this tool is appropriate for the tools-iuc repo.
• License permits unrestricted use (educational + commercial)
• This PR adds a new tool or tool collection
• This PR updates an existing tool or tool collection
• This PR does something else (explain below)

[bgruening]

@bernt-matthias I think this only works for ITs at the moment.

[davelopez]

Yes, @bgruening is right, the icon is only displayed in interactive tools for now. But if we want to eventually use it for every tool, it's good to have it already beside the XML :)

[bernt-matthias]

Thanks for the clarification.