Add SPDX identifier to source code

Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>

src/components.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 use std::{
     collections::HashMap,
     fs::{File, OpenOptions},

src/disk.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 use std::io::{Read, Write};
 use std::{fs::File, path::Path};
 

src/main.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 use std::collections::HashMap;
 use std::error;
 use std::fmt;