Changed stream from stderr to stdout for successful processing

[darkclouder]

Successful processing of files spams stderr, should be stdout because it's no error.
It also makes it easier to filter those by redirecting it to /dev/null

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[darkclouder]

I signed it!

[googlebot]

CLAs look good, thanks!

[tkruse]

Consider submitting the patch to the fork at https://github.com/cpplint/cpplint