file-sys: Default heavy-weight class destructors in the cpp file

Several classes have a lot of non-trivial members within them, or don't
but likely should have the destructor defaulted in the cpp file for
future-proofing/being more friendly to forward declarations.

Leaving the destructor unspecified allows the compiler to inline the
destruction code all over the place, which is generally undesirable from
a code bloat perspective.

src/core/file_sys/vfs_offset.h

@@ -19,6 +19,7 @@ class OffsetVfsFile : public VfsFile {
 public:
     OffsetVfsFile(std::shared_ptr<VfsFile> file, std::size_t size, std::size_t offset = 0,
                   std::string new_name = "", VirtualDir new_parent = nullptr);
+    ~OffsetVfsFile() override;
 
     std::string GetName() const override;
     std::size_t GetSize() const override;