Fix generated wrapper for nested struct with static fn

[roife]

This PR fixes an issue with the wrappers for static functions in nested structs.

See the added test case:

struct A {
    struct B {
        static void f() {}
    };
};

Currently, autocxx would generates erroneous code namespace::A_B::f() instead of namespace::A::B::f().

[adetaylor]

Thanks for this - I'm unclear why the github CI hasn't run the full suite of tests against it, and I don't want to merge it without that.

[adetaylor]

I raised another PR for the same code to get tests running - #1387 - there's a failure which needs looking into.