Fingerprint: add new signature method

commit c42a54795d introcuded a bug by
using fp.to_string(). Replace this with fp.signature() which correctly
returns the full fingerprint instead of the short version.

Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>

pbs-api-types/src/crypto.rs

@@ -33,6 +33,9 @@ impl Fingerprint {
     pub fn bytes(&self) -> &[u8; 32] {
         &self.bytes
     }
+    pub fn signature(&self) -> String {
+        as_fingerprint(&self.bytes)
+    }
 }
 
 /// Display as short key ID