GPT: add accessor function for disk GUID

In order to read the GPT, modify the partition name strings, and then
write out a new GPT, the disk GUID is needed.  While there is an
existing accessor for the partition UUIDs, there is none yet for the
disk GUID.

Changes since v6: none.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>
diff --git a/include/part.h b/include/part.h
index 22da604..c41aa6a 100644
--- a/include/part.h
+++ b/include/part.h
@@ -372,6 +372,21 @@
 int gpt_verify_partitions(struct blk_desc *dev_desc,
 			  disk_partition_t *partitions, int parts,
 			  gpt_header *gpt_head, gpt_entry **gpt_pte);
+
+
+/**
+ * get_disk_guid() - Function to read the GUID string from a device's GPT
+ *
+ * This function reads the GUID string from a block device whose descriptor
+ * is provided.
+ *
+ * @param dev_desc - block device descriptor
+ * @param guid - pre-allocated string in which to return the GUID
+ *
+ * @return - '0' on success, otherwise error
+ */
+int get_disk_guid(struct blk_desc *dev_desc, char *guid);
+
 #endif
 
 #if CONFIG_IS_ENABLED(DOS_PARTITION)