Fix REUSE compliance for fuzz/.gitignore

[RAprogramm]

Fix missing SPDX header in fuzz/.gitignore to comply with REUSE 3.3 specification.

Problem

CI fails on REUSE compliance check:

The following files have no copyright and licensing information:
* fuzz/.gitignore

Solution

Add SPDX header to fuzz/.gitignore:

# SPDX-FileCopyrightText: 2025 RAprogramm <andrey.rozanov.vl@gmail.com>
# SPDX-License-Identifier: CC0-1.0

Impact

• Blocks all PRs from merging
• Main branch CI failing