Asio integration: link error in yield.hpp included in different cpp

In the file `examples/asio/yield.hpp` there is the line
```
thread_local yield_t yield{};
```
So if this yield.hpp included in two different cpp files then there will be link issue.

If existing code will be changed with following then issue is solved.
```
inline yield_t& this_yield() {
thread_local yield_t yield;
return yield;
}
```