fix: Change import of 'flat' library

[jnain]

PR Checklist

Please check if your PR fulfills the following requirements:

• [X] The commit message follows our guidelines: https://github.com/ngneat/transloco/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: https://github.com/ngneat/transloco/issues/587

What is the new behavior?

My app builds with my modif but your lib transloco-schematics does not build anymore. And I don't really know why :(

Does this PR introduce a breaking change?

[ ] Yes 
[X] No (because one of the build step is broken)

Other information