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_concat.h

@@ -22,6 +22,8 @@ class ConcatenatedVfsFile : public VfsFile {
     ConcatenatedVfsFile(std::vector<VirtualFile> files, std::string name);
 
 public:
+    ~ConcatenatedVfsFile() override;
+
     std::string GetName() const override;
     std::size_t GetSize() const override;
     bool Resize(std::size_t new_size) override;