Fix swagger generation

The swagger files should be generated by the `make proto-swagger-gen` command, but it fails:

```
$ make proto-swagger-gen
Generating Protobuf Swagger
Failure: could not read file buf.gen.swagger.yaml: open buf.gen.swagger.yaml: no such file or directory
make: *** [Makefile:338: proto-swagger-gen] Error 1
```

The same error is triggered with Gaia v15.2.0 where this repo is forked from.