pw_ring_buffer: Fix span dependency
pw_ring_buffer: Fix span dependency

The span dependency needs to be public since it's used in the public
API.

Change-Id: Ibf148b15b11108526ff11e1e4b15f4ef84237ae8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100183
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed