FILE_TRUNC does not exist on Solaris

This does not compile on Solaris due to C.FILE_TRUNC missing.

`.../go/pkg/mod/github.com/syncthing/notify@v0.0.0-20210308121556-f45149b04939/watcher_fen_cgo.go:30:25: could not determine kind of name for C.FILE_TRUNC`