Doesn't build on Darwin with Go 1.11beta3

[Jakob Borg]

So the old issue from #135 (that was introduced in Go 1.10beta1 and then reverted in a later beta) has resurfaced in Go 1.11beta1 and remains in 1.11beta3. The issue is this:

$ go test
# github.com/rjeczalik/notify [github.com/rjeczalik/notify.test]
./watcher_fsevents_cgo.go:51:216: cannot use nil as type _Ctype_CFAllocatorRef in argument to func literal
./watcher_fsevents_cgo.go:169:47: cannot use nil as type _Ctype_CFAllocatorRef in argument to _Cfunc_CFStringCreateWithCStringNoCopy
./watcher_fsevents_cgo.go:170:225: cannot use nil as type _Ctype_CFAllocatorRef in argument to func literal
FAIL	github.com/rjeczalik/notify [build failed]

I'm sure the old patch, applied again, would solve it this time as well. Of course, it could be reverted again in a newer beta (who knows, it happened before). Nonetheless it would be nice to be able to build stuff with the 1.11 betas in the meantime, I think. :/

(I'll file a PR.)