tools: list missing whitespace/if-one-line cpplint

Writing `// NOLINT(whitespace/if-one-line)` was not possible because the
directive was not listed in the list of known lint rules.  You can now.

PR-URL: #4099
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/cpplint.py

@@ -199,6 +199,7 @@
   'whitespace/comments',
   'whitespace/end_of_line',
   'whitespace/ending_newline',
+  'whitespace/if-one-line',
   'whitespace/indent',
   'whitespace/labels',
   'whitespace/line_length',