GCZ: use 64-bit for m_data_offset

A malicious GCZ file could probably force this to be negative.
Shouldn't cause any issues other than file read failures, but need to fix
because it is causing errors on MSVC.

Source/Core/DiscIO/CompressedBlob.h

@@ -72,7 +72,7 @@ private:
   CompressedBlobHeader m_header = {};
   std::vector<u64> m_block_pointers = {};
   std::vector<u32> m_hashes = {};
-  int m_data_offset = 0;
+  u64 m_data_offset = 0;
   File::DirectIOFile m_file = {};
   u64 m_file_size = 0;
   std::vector<u8> m_zlib_buffer = {};