Adding forgotten duration.proto to the lite library

[robcos]

timestamp.proto is part of the java lite release but duration is not.
I assume this is accidental.

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[robcos]

@googlebot I signed it!

[robcos]

release notes: yes

[acozzette]

Thanks, @robcos.

[robcos]

This commit fixes #7094

[robcos]

@acozzette (I think I met you but don't know where :-)) Do you have an idea of when this is going to be released?

[acozzette]

I think we might have met at the YouTube office in San Bruno one time. I'm working on a release now so let me just cherry-pick this since it is a small fix.