qtprotobufgen: header guards with invalid C-identifier

We generate two ways for the include guards: pragma and header guards. The latter approach has some problems though as it struggles with dashes "-" in the filename, e.g. "my-proto.proto". We could transform this into a valid C-identifier, e.g. "MY_PROTO..." but how would we handle the case where we have "my-proto.proto" and "my_proto.proto" in the same project?

We should either fix the header guards approach for this edge case or switch to "#pragma once" as the default and deprecate header guards in the upcoming releases. Current state with a partly broken default is not nice.