Pass --root to cpplint.

cpplint looks for .git to detect root, but in gitc, there's no art/.git
unless I repo start in art/ first, but I'm not working on art and
checkbuild fails in gitc because of this.

Bug: None
Test: mma in art/ in gitc

Change-Id: I6d418979a9bdceefc7a2c19297a7cc9aacb3e83f