cloud/storage/dropbox: fix build
cloud/storage/dropbox: fix build

Change-Id: I46deb0a9b71fa5bb770eed8fd79399f073041529
Reviewed-on: https://upspin-review.googlesource.com/14700
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/dropbox