[build] Return const char * from uuid_ntoa()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/uuid.c

  * @v uuid		UUID
  * @ret string		UUID in canonical form
  */
-char * uuid_ntoa ( const union uuid *uuid ) {
+const char * uuid_ntoa ( const union uuid *uuid ) {
 	static char buf[37]; /* "00000000-0000-0000-0000-000000000000" */
 
 	sprintf ( buf, "%08x-%04x-%04x-%04x-%02x%02x%02x%02x%02x%02x",

src/include/ipxe/uuid.h

 	__bswap_16s ( &uuid->canonical.c );
 }
 
-extern char * uuid_ntoa ( const union uuid *uuid );
+extern const char * uuid_ntoa ( const union uuid *uuid );
 
 #endif /* _IPXE_UUID_H */

src/net/peerdisc.c

 	} random_uuid;
 	size_t uuid_len;
 	size_t id_len;
-	char *uuid;
+	const char *uuid;
 	char *uuid_copy;
 	char *id_copy;
 	unsigned int i;