Add GPL v2 license grant to mysql CNID backend code

[rdmark]

This adds the standard GNU GPL v2 license grant blurb to this one source file. It currently defers to COPYING for the license, which is ambiguous.

Note: Awaits approval from the original author.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rdmark]

@slowfranklin As the author of this code, would you agree to this license grant for your work?

This would align the grant for this source file with all the rest of the GPLv2 licensed C code in this repository, and remove licensing ambiguity.