C++Qt blocks allow for #[inherit] attribute on signals when it is unused and has no purpose there

[ahayzen-kdab]

Currently you can write the following

unsafe extern "C++Qt" {
  type ExternObject;

  #[qsignal]
  #[inherit]
  fn method(self: &ExternObject);
}

Signals from C++Qt blocks do not have any difference from the inherit attribute, so likely we should disallow the attribute here?

Note the reason this currently works is that the parser code for signals is shared between RustQt and C++Qt blocks.