build-sys: Print libsodium status at end of configure

Like we do with other features.

configure.ac

@@ -641,6 +641,7 @@ echo "
     cryptographic checksums:                      $with_crypto
     systemd:                                      $with_libsystemd
     libmount:                                     $with_libmount
+    libsodium (ed25519 signatures):               $with_libsodium
     libarchive (parse tar files directly):        $with_libarchive
     static deltas:                                yes (always enabled now)
     O_TMPFILE:                                    $enable_otmpfile