Add Definition to Auditd susp_activity

[BalsamicSentry]

I'd like to suggest adding a definition field to an Auditd rule that requires specific rules to be applied.

I see that the reference and description of the rule mention that the Auditd rule is custom, but I think it would be more clear if definition was added.

This is my first issue, sorry if I do something wrong

[github-actions]

Welcome @BalsamicSentry 👋

It looks like this is your first issue on the Sigma rules repository!

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Thanks for taking the time to open this issue, and welcome to the Sigma community! 😃