Add license checking github action

[mithro]

https://github.com/SymbiFlow/actions/tree/main/checks

[mithro]

Example at https://github.com/SymbiFlow/symbiflow-rr-graph/blob/master/.github/workflows/licensing.yml

GitHub

SymbiFlow/symbiflow-rr-graph

Collection of Routing Resources Graph (RR Graph) libraries for VPR - SymbiFlow/symbiflow-rr-graph

[duck2]

So I add a license header to all files and add a github action to check for that? Would be good to move CI to github actions while at it.