Enable GitLab CI/CD
Deploys both a native binary to RiscOS/Library and a cross-compiling one to the GitLab runner machine.
Fix issues identified by CI:
- Several header files were missing copyright notices and licence terms: copy these from the corresponding source files.
- Tell GitLab that one source file is yacc format (rouge syntax colour library doesn't know yacc, so select C as next-best fit).
Requires RiscOS/Sources/Lib/OSLib!2 (merged).