spaces_inside_linter doesn't ignore trailing comments

Found in the course of working on #598 

```
writeLines("
lm( # cool regression
 y ~ x
)
", tmp <- tempfile())

lintr::lint(tmp, lintr::spaces_inside_linter)
# /tmp/RtmpAD3BJR/file41eb53c52da:2:4: style: Do not place spaces around code in parentheses or square brackets.
# lm( # cool regression
#    ^
```

trailing whitespace should be ignored if there is no code after the the `(`.