core/debugger: Define defaulted virtual destructors

Resolves an MSVC warning where a virtual destructor is not defined in the base class with virtual functions.

src/core/debugger/gdbstub.h

@@ -19,7 +19,7 @@ class System;
 class GDBStub : public DebuggerFrontend {
 public:
     explicit GDBStub(DebuggerBackend& backend, Core::System& system);
-    ~GDBStub();
+    ~GDBStub() override;
 
     void Connected() override;
     void Stopped(Kernel::KThread* thread) override;