Disable lvalue access to GThread types

Otherwise a copy will be made whenever such a value is passed as a method
argument, and this is really bad in the case of a GMutex. (Where the lock
may be inline in the struct.)

Here's a test-case demonstrating bad code being generated:

```Vala
bool finished = false;
Mutex mutex;
Cond cond;

int main (string[] args) {
	mutex.lock ();
	while (!finished) {
		cond.wait (mutex);
	}
	mutex.unlock ();
	return 0;
}
```