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/partition_filesystem.h

@@ -25,6 +25,8 @@ namespace FileSys {
 class PartitionFilesystem : public ReadOnlyVfsDirectory {
 public:
     explicit PartitionFilesystem(std::shared_ptr<VfsFile> file);
+    ~PartitionFilesystem() override;
+
     Loader::ResultStatus GetStatus() const;
 
     std::vector<std::shared_ptr<VfsFile>> GetFiles() const override;