Fix compact npos solution edge count calculation (#7021)

This edge count is used for weighing, and it is somewhat trivial to review and verify that the current implementation was ignoring `votes16` field of the struct. As reminder, the struct is like this: 
```rust

struct Compact {
  votes1: ... ,
  votes2: ..., 
  ...,
  votes16: ...,
}
```

I already will fix this in https://github.com/paritytech/substrate/pull/7007, but since it might take a while, this one can go in asap and make it to the very next runtime.