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/xts_archive.h

@@ -33,6 +33,7 @@ class NAX : public ReadOnlyVfsDirectory {
 public:
     explicit NAX(VirtualFile file);
     explicit NAX(VirtualFile file, std::array<u8, 0x10> nca_id);
+    ~NAX() override;
 
     Loader::ResultStatus GetStatus() const;