video_core/memory_manager: Mark the constructor as explicit

Prevents implicit converting constructions of the memory manager.

src/video_core/memory_manager.h

@@ -47,7 +47,7 @@ struct VirtualMemoryArea {
 
 class MemoryManager final {
 public:
-    MemoryManager(VideoCore::RasterizerInterface& rasterizer);
+    explicit MemoryManager(VideoCore::RasterizerInterface& rasterizer);
     ~MemoryManager();
 
     GPUVAddr AllocateSpace(u64 size, u64 align);