Add gcc 7.4 to settings.yml

[Hopobcn]

Changelog: Feature: Add support for gcc 7.4.
Docs: conan-io/docs#1438

Closes #5890

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^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

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