Add SPDX License Identifiers to all source file headers

[leogr]

Motivation

As part of our continuous effort to align with CNCF IP Policy, we requested a license scan. This scan revealed no outstanding issues, but there were a couple of recommended action items.

One reported action item is:

Action Item 2: Add SPDX License Identifiers to all source file headers.

• We recommend that SPDX license identifiers be added to ALL source file headers. [see https://spdx.dev/ids for examples]

#### Action item
- [x] Add SPDX License Identifiers to all source file headers in all repositories
- [ ] https://github.com/falcosecurity/.github/pull/117
- [ ] https://github.com/falcosecurity/falco/pull/2809
- [ ] https://github.com/falcosecurity/libs/pull/1387
- [ ] https://github.com/falcosecurity/libs/pull/1390
- [ ] https://github.com/falcosecurity/falcoctl/pull/330
- [ ] https://github.com/falcosecurity/plugin-sdk-go/pull/82
- [ ] https://github.com/falcosecurity/plugins/pull/362
- [ ] https://github.com/falcosecurity/rules/pull/183
- [ ] https://github.com/falcosecurity/falcosidekick/pull/664
- [ ] https://github.com/falcosecurity/driverkit/pull/294
- [ ] https://github.com/falcosecurity/event-generator/pull/87
- [ ] https://github.com/falcosecurity/falco-exporter/pull/94
- [ ] https://github.com/falcosecurity/falco-aws-terraform/pull/5
- [ ] https://github.com/falcosecurity/falcosidekick-ui/pull/125
- [ ] https://github.com/falcosecurity/flycheck-falco-rules/pull/5
- [ ] https://github.com/falcosecurity/plugin-sdk-cpp/pull/26
- [ ] https://github.com/falcosecurity/test-infra/pull/1254
- [ ] https://github.com/falcosecurity/dbg-go/pull/29
- [ ] https://github.com/falcosecurity/pigeon/pull/16
- [ ] https://github.com/falcosecurity/testing/pull/32
- [ ] https://github.com/falcosecurity/kernel-crawler/pull/178
- [ ] https://github.com/falcosecurity/syscalls-bumper/pull/17
- [ ] https://github.com/falcosecurity/kernel-testing/pull/73
- [ ] https://github.com/falcosecurity/falco-playground/pull/33
- [ ] https://github.com/falcosecurity/evolution/pull/327

[poiana]

Issues go stale after 90d of inactivity.

Mark the issue as fresh with /remove-lifecycle stale.

Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Provide feedback via https://github.com/falcosecurity/community.

/lifecycle stale

[leogr]

This is done.