src/backup.rs; use a macro to define PROXMOX_BACKUP_PROTOCOL_ID_V1

So that we can include it in static doc strings.

src/backup.rs

@@ -102,7 +102,10 @@
 //!
 //! Not sure if this is better. TODO
 
-pub const PROXMOX_BACKUP_PROTOCOL_ID_V1: &str = "proxmox-backup-protocol-v1";
+#[macro_export]
+macro_rules! PROXMOX_BACKUP_PROTOCOL_ID_V1 {
+    () =>  { "proxmox-backup-protocol-v1" }
+}
 
 mod chunk_stream;
 pub use chunk_stream::*;