enable fsnotify

[simonmichael]

Checklist:

• Meaningful commit message - please not Update build-constraints.yml

• At least 30 minutes have passed since Hackage upload

• On your own machine, in a new directory, you have successfully run the following set of commands (replace $package with the name of the package that is submitted, $version is the version of the package you want to get into Stackage):

  stack unpack $package
  cd $package-$version
  stack init --resolver nightly
  stack build --resolver nightly --haddock --test --bench --no-run-benchmarks
  

[simonmichael]

Oops, this is related to #3501

[alexeyzab]

Thanks!