add various nodiscard

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/event/ServerSocket.cxx

@@ -91,7 +91,7 @@ public:
 	}
 #endif
 
-	bool IsDefined() const noexcept {
+	[[nodiscard]] bool IsDefined() const noexcept {
 		return event.IsDefined();
 	}
 

src/event/SignalMonitor.cxx

@@ -62,7 +62,7 @@ public:
 #endif
 	}
 
-	auto &GetEventLoop() const noexcept {
+	[[nodiscard]] auto &GetEventLoop() const noexcept {
 		return event.GetEventLoop();
 	}