findmissing: Add v5.8 to list of stable branches

BUG=None
TEST=Run tool and verify that 5.8 is supported

Change-Id: Iab4ca19d77f0cef0fba19adf8f821b90e8e6dec0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2364469
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
Commit-Queue: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>