Fix DNS filter in CMakeLists file

[Tempate]

Description

In the previous version, the DNS Filter tests were not being compiled when the tag EPROSIMA_TEST_DNS_NOT_SET_UP was set.

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• NA Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• NA Any new/modified methods have been properly documented using Doxygen.
• Changes are ABI compatible.
• Changes are API compatible.
• NA New feature has been added to the versions.md file (if applicable).
• NA New feature has been documented/Current behavior is correctly described in the documentation.
• N/A Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.