Unable to build example

Looks like problem with build tag.

```shell
$ tinygo flash -target=pca10040-s132v6 ./examples/heartrate
# github.com/aykevl/go-bluetooth
adapter.go:7:10: undeclared name: Adapter
```

If I modify the various `_sd` files to only be:

```go
// +build softdevice
```

Then I can build as expected.