Software/SWGfx: Default destructor in cpp file

Fixes a build failure with clang 17.

The destructor needs to be in the cpp file, since we have a forward
declared std::unique_ptr type as part of the class. So technically the
default inline destructor could invoke without seeing the full data type
definition.

Source/Core/VideoBackends/Software/SWGfx.cpp

@@ -22,6 +22,8 @@ SWGfx::SWGfx(std::unique_ptr<SWOGLWindow> window) : m_window(std::move(window))
 {
 }
 
+SWGfx::~SWGfx() = default;
+
 bool SWGfx::IsHeadless() const
 {
   return m_window->IsHeadless();