QUIC: fix: forward-declare Indication in AppSocket.h (#1107)

AppSocket.h uses inet::Indication only through pointers
(sendIndication, std::list<Indication*>) but did not name it,
relying on a deep transitive include chain. This broke on some
configurations. Add a forward declaration in the inet namespace.

Author: avarga

src/inet/transportlayer/quic/AppSocket.h

@@ -13,6 +13,7 @@
 #include "Quic.h"
 
 namespace inet {
+class Indication;   // inet::Indication (used only via pointer below)
 namespace quic {
 
 // Forward declarations: