video_core/memory_manager: Default the destructor within the cpp file

Makes the class less surprising when it comes to forward declaring the
type, and also prevents inlining the destruction code of the class,
given it contains non-trivial types.

src/video_core/memory_manager.h

@@ -48,6 +48,7 @@ struct VirtualMemoryArea {
 class MemoryManager final {
 public:
     MemoryManager(VideoCore::RasterizerInterface& rasterizer);
+    ~MemoryManager();
 
     GPUVAddr AllocateSpace(u64 size, u64 align);
     GPUVAddr AllocateSpace(GPUVAddr addr, u64 size, u64 align);