core/frontend/emu_window: Make GraphicsContext's destructor virtual

This class is used in a polymorphic context, so destruction of the
context will lead to undefined behavior if the destructor isn't virtual.

src/core/frontend/emu_window.cpp

@@ -10,6 +10,8 @@
 
 namespace Core::Frontend {
 
+GraphicsContext::~GraphicsContext() = default;
+
 class EmuWindow::TouchState : public Input::Factory<Input::TouchDevice>,
                               public std::enable_shared_from_this<TouchState> {
 public: