Add back poll of change signaller after adding starting fd monitor

This was missed in ce559bc20 (Port fd_monitor (and its needed components),
2023-02-17).

Fixes the wrong color in

    echo (set_color yellow; date +%T; set_color blue)

as reported in https://github.com/fish-shell/fish-shell/issues/10176#issuecomment-1872658589

fish-rust/src/fd_monitor.rs

@@ -517,6 +517,9 @@ pub fn add(&self, mut item: FdMonitorItem) -> FdMonitorItemId {
             });
         }
 
+        // Tickle our signaller.
+        self.change_signaller.post();
+
         item_id
     }