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/nca_metadata.h

@@ -87,6 +87,7 @@ public:
     explicit CNMT(VirtualFile file);
     CNMT(CNMTHeader header, OptionalHeader opt_header, std::vector<ContentRecord> content_records,
          std::vector<MetaRecord> meta_records);
+    ~CNMT();
 
     u64 GetTitleID() const;
     u32 GetTitleVersion() const;