yuzu/configuration/configure_input: Default destructor in the cpp file

The previous code could potentially be a compilation issue waiting to
occur, given we forward declare the type for a std::unique_ptr. If the
complete definition of the forward declared type isn't visible in a
translation unit that the class is used in, then it would fail to
compile.

Defaulting the destructor in a cpp file ensures the std::unique_ptr's
destructor is only invoked where its complete type is known.

src/citra_qt/configuration/configure_input.h

@@ -30,6 +30,7 @@ class ConfigureInput : public QWidget {
 
 public:
     explicit ConfigureInput(QWidget* parent = nullptr);
+    ~ConfigureInput() override;
 
     /// Save all button configurations to settings file
     void applyConfiguration();