Compiler error when using @:nativeGen on interface and an implementing class on C++ target

The issue appears to be in generating the inheritance list. To reproduce:
Test.hx:
```
@:nativeGen
interface ITest {
public function foo():Void;
}

@:nativeGen
class TestImpl implements ITest {
public function foo() {}
}
```
Compile with:
`haxe -cpp test Test.hx`

This produces the following class definition in TestImpl.h:
```
class HXCPP_CLASS_ATTRIBUTES TestImpl , public virtual ITest
{
public:
void foo();

};
```

It looks like the error might be in gencpp.ml, in generate_class_header. dump_native_interfaces always emits a , first, but is called from a branch of code that will not emit an initial base class (a few lines below, `else if (super=="") then...`).

It seems like this only affects native classes implementing native interfaces (removing @:nativeGen from ITest compiles, but TestImpl loses that inheritance).

Happy to submit a PR with a patch.

Start with gencpp.ml, especially generate_class_header and dump_native_interfaces. Reproduce the issue with the Test.hx example and `haxe -cpp test Test.hx`, then inspect the generated TestImpl.h. Done means the C++ target compiles for native classes implementing native interfaces and emits a valid inheritance list.

Written by the indexing model from the issue text.