Fix Darwin build without Cgo

[tie]

Either cross-compiling or explicit CGO_ENABLED=0:

GOOS=darwin go build

Expected output: builds without an error
Actual output:

# github.com/rjeczalik/notify
./watcher_fsevents.go:49:11: undefined: stream

This PR addresses this issue by adding a fallback to kqueue on darwin,!cgo and enabling fsevents only for darwin,!kqueue,cgo.

[dvaldivia]

This looks great, when can this get merged?

[rjeczalik]

Thanks @tie for the PR!