Review Request: bool stop() -> void stop() in process::Executor

[Charles Reiss]

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https://reviews.apache.org/r/4664/
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Review request for mesos and Benjamin Hindman.


Summary
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See summary. Fixes a GCC warning.


Diffs
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  third_party/libprocess/include/process/executor.hpp 7ac295f 

Diff: https://reviews.apache.org/r/4664/diff


Testing
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Thanks,

Charles