Add dual-license notice (corrected fork)

[AkarshSahlot]

Changes I have done:

This PR updates the LICENSE file inside p4c to explicitly:

1. State that p4c is primarily apache2.0 licensed
2. List all gpl2 only files(with full license text)
3. Add warnings about license incompatibility
4. Include SPDX identifier (Apache-2.0 WITH GPL-2.0-only-exception) so that users doesn't get confuse project as pure Apache, ignoring GPL restrictions.

Previously Behavior:

The repository only listed Apache-2.0 in the top-level LICENSE file, which make users think it's purely apache
gpl2-only files existed but were not clearly documented , which has a risk of:

• Unintentional license violations when combining code
• Misrepresentation of the project’s licensing (GitHub’s API showed only Apache-2.0)

Why This Important for us to do:

Prevents accidental mixing of incompatible licenses.

Increased Transparency, Users/tools can now identify GPL-2.0-only files at a glance.

Now it follows spdx standards for multi-license projects.
Verification

All listed GPL files exist (./backends/ebpf/tests/ptf/*.py)

License texts are verbatim from official sources
Compliance notes
warn against static linking/binary distribution

I verified them locally

This small addition solves big legal/automation headaches!


/cc @jafingerhut
Could you please review this license clarification?
Fixes #666

[jafingerhut]

Same comments as here: p4lang/p4c#5212 (comment)

[AkarshSahlot]

Will wait for it