flow: constify getters param

Some potential callers are already using constified values so it
is good to do it.

src/flow.c

@@ -860,12 +860,12 @@ int FlowSetProtoEmergencyTimeout(uint8_t proto, uint32_t emerg_new_timeout,
     return 1;
 }
 
-AppProto FlowGetAppProtocol(Flow *f)
+AppProto FlowGetAppProtocol(const Flow *f)
 {
     return f->alproto;
 }
 
-void *FlowGetAppState(Flow *f)
+void *FlowGetAppState(const Flow *f)
 {
     return f->alstate;
 }

src/flow.h

@@ -574,8 +574,8 @@ static inline void FlowDeReference(Flow **d)
 
 int FlowClearMemory(Flow *,uint8_t );
 
-AppProto FlowGetAppProtocol(Flow *f);
+AppProto FlowGetAppProtocol(const Flow *f);
-void *FlowGetAppState(Flow *f);
+void *FlowGetAppState(const Flow *f);
 
 void FlowHandlePacketUpdateRemove(Flow *f, Packet *p);
 void FlowHandlePacketUpdate(Flow *f, Packet *p);