add handlers constness

src/main.cpp

@@ -4,16 +4,16 @@
 #include <memory>
 
 class KeepAliveHandler : public sp::PacketHandler {
-	void Handle(KeepAlivePacket& packet) {
+	void Handle(const KeepAlivePacket& packet) {
 		std::cout << "KeepAlive handled !\n";
 	}
 
-	void Handle(DisconnectPacket& packet) {
+	void Handle(const DisconnectPacket& packet) {
 		std::cout << "Disconnect handled !\n";
 	}
 
 	// TODO: add constness
-	void Handle(UpgradeTowerPacket& packet) {
+	void Handle(const UpgradeTowerPacket& packet) {
 		std::cout << "UpgradeTower handled !\n";
 	}
 };