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ARROW-3361: [R] Also run cpplint on Rcpp source files

Also fix a couple of lint issues that the script turned up

Author: Wes McKinney <wesm+git@apache.org>

Closes #3773 from wesm/ARROW-3361 and squashes the following commits:

9c390514 <Wes McKinney> Also run cpplint on Rcpp source files