Harmonize license/copyright with Rizin with SPDX

[XVilka]

Use the same scheme as Rizin does. For example, header could look like this:

//  SPDX-FileCopyrightText: 2019 Jane Doe <jane@example.com>
//  SPDX-License-Identifier: GPL-3.0-or-later

More at https://reuse.software/faq/#quick-summary

See also:

• https://github.com/rizinorg/rizin/issues/683
• https://github.com/rizinorg/rizin/issues/3520

[TAbhay]

I would like to work on it

[XVilka]

Sure, you can see how it had been done in Rizin itself, and base it on the git history.