DiscIO: Add way of checking whether blob data size is accurate

Source/Core/DiscIO/CISOBlob.h

@@ -40,6 +40,7 @@ public:
   // The CISO format does not save the original file size.
   // This function returns an upper bound.
   u64 GetDataSize() const override;
+  bool IsDataSizeAccurate() const override { return false; }
 
   u64 GetRawSize() const override;
   bool Read(u64 offset, u64 nbytes, u8* out_ptr) override;