workflows: Ignore checkpatch UNKNOWN_COMMIT_ID

As we do a shallow clone of the repo, Fixes: tags
generally don't have the matching commit available
to lookup, and checkpatch logs it.

Ignore this error.

Signed-off-by: Dave Stevenson <[email protected]>

Author: 6by9

.github/workflows/ci_checkpatch.conf

@@ -2,3 +2,4 @@
 --ignore FILE_PATH_CHANGES
 --ignore GIT_COMMIT_ID
 --ignore SPDX_LICENSE_TAG
+--ignore UNKNOWN_COMMIT_ID