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.cpp

@@ -51,6 +51,8 @@ CNMT::CNMT(CNMTHeader header, OptionalHeader opt_header, std::vector<ContentReco
     : header(std::move(header)), opt_header(std::move(opt_header)),
       content_records(std::move(content_records)), meta_records(std::move(meta_records)) {}
 
+CNMT::~CNMT() = default;
+
 u64 CNMT::GetTitleID() const {
     return header.title_id;
 }