LibCore: Stub out FileWatcher for Lagom platforms

Stub out the FileWatcher class with ENOTSUP stubs to let Services that
require it to compile for Lagom. Later we should add real code for this
using techniques like Linux's inotify(7).

Userland/Libraries/LibCore/FileWatcher.cpp

@@ -206,6 +206,30 @@ FileWatcher::~FileWatcher()
     dbgln_if(FILE_WATCHER_DEBUG, "Stopped watcher at fd {}", m_notifier->fd());
 }
 
+#else
+// FIXME: Implement Core::FileWatcher for linux, macOS, and *BSD
+FileWatcher::~FileWatcher() { }
+FileWatcher::FileWatcher(int watcher_fd, NonnullRefPtr<Notifier> notifier)
+    : FileWatcherBase(watcher_fd)
+    , m_notifier(move(notifier))
+{
+}
+
+ErrorOr<NonnullRefPtr<FileWatcher>> FileWatcher::create(InodeWatcherFlags)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
+ErrorOr<bool> FileWatcherBase::add_watch(String, FileWatcherEvent::Type)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
+ErrorOr<bool> FileWatcherBase::remove_watch(String)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
 #endif
 
 }