zeroconf/Bonjour: fix OnSocketReady() return value

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8a0, andd
apparently, nobody ever noticed.

src/zeroconf/ZeroconfBonjour.cxx

@@ -50,7 +50,7 @@ protected:
 	/* virtual methods from class SocketMonitor */
 	bool OnSocketReady(gcc_unused unsigned flags) noexcept override {
 		DNSServiceProcessResult(service_ref);
-		return false;
+		return true;
 	}
 };