Ignore generated linter.log

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1577

.gitignore

@@ -133,6 +133,7 @@ _darcs/
 /libraries/plus.gif
 /libraries/synopsis.png
 /libraries/stamp/
+/linter.log
 /mk/are-validating.mk
 /mk/build.mk
 /mk/config.h