[scudo] Remove internal linkage from LargeBlock header helpers (NFC) (#207708) `addHeaderTag` and `getHeader` are static function templates in `secondary.h,` so any TU that includes the header without using them trips `-Wunused-template`. Dropping static fixes the warning. Part of #202945. GitOrigin-RevId: dcf969eaf2c25048542d13946ca5ff65be5bc7ab