SVEventHandler: Define virtual destructor in .cpp file

This fixes compiler warnings from clang:

src/viewer/scrollview.h:86:7: warning:
 'SVEventHandler' has no out-of-line virtual method definitions;
 its vtable will be emitted in every translation unit [-Wweak-vtables]

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/viewer/scrollview.cpp

@@ -73,6 +73,8 @@ SVEvent* SVEvent::copy() {
   return any;
 }
 
+SVEventHandler::~SVEventHandler() = default;
+
 #ifndef GRAPHICS_DISABLED
 /// This is the main loop which handles the ScrollView-logic from the server
 /// to the client. It basically loops through messages, parses them to events

src/viewer/scrollview.h

@@ -85,7 +85,7 @@ struct SVEvent {
 // called whenever an appropriate event occurs.
 class SVEventHandler {
   public:
-    virtual ~SVEventHandler() {}
+    virtual ~SVEventHandler();
 
 // Gets called by the SV Window. Does nothing on default, overwrite this
 // to implement the desired behaviour