GPT: Fix creation of GPT disk systems; resolves #10298

* The alternate / backup block wasn't defined
  leading to an invalid configuration.
* This was compounded with another issue that
  was resolved a while back with GPT
* gdisk now identifies GPT systems created with
  Haiku as valid

src/add-ons/kernel/partitioning_systems/gpt/Header.cpp

@@ -137,7 +137,7 @@ Header::Header(uint64 lastBlock, uint32 blockSize)
 	fHeader.SetHeaderSize(sizeof(fHeader));
 	fHeader.SetHeaderCRC(0);
 	fHeader.SetAbsoluteBlock(EFI_HEADER_LOCATION);
-	fHeader.SetAlternateBlock(0); // TODO
+	fHeader.SetAlternateBlock(lastBlock);
 	// TODO: set disk guid
 	fHeader.SetEntriesBlock(EFI_PARTITION_ENTRIES_BLOCK);
 	fHeader.SetEntryCount(EFI_PARTITION_ENTRY_COUNT);