ipn/ipnlocal: fix regression in printf arg type

I screwed this up in 58a4fd43d as I expected. I even looked out for
cases like this (because this always happens) and I still missed
it. Vet doesn't flag these because they're not the standard printf
funcs it knows about. TODO: make our vet recognize all our
"logger.Logf" types.

Updates #8948

Change-Id: Iae267d5f81da49d0876b91c0e6dc451bf7dcd721
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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