utils/generate-cyclonedx: make urllib.error import explicit

Commit [1] introduced a code block that references
'urllib.error.URLError'. There is no explicit import of 'urllib.error'.
While this is not an issue in practice, ruff complains that the import
is not explicit.

[1] 17c9deb115 utils/generate-cyclonedx: hint at missing Buildroot host package on a specific error

Signed-off-by: Thomas Perale <thomas.perale@mind.be>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

utils/generate-cyclonedx

@@ -14,6 +14,7 @@ import json
 import os
 from pathlib import Path
 from typing import Iterator
+import urllib.error
 import urllib.parse
 import urllib.request
 import subprocess