Untracked headers fly under the 'gn check' radar and in the long term
this can cause problems like unnoticed cyclic dependencies.
This cl creates a synthetic target for this header since no other
targets in webrtc/modules/pacing/BUILD.gn seem to be related to it.
BUG=webrtc:7649
NOTRY=True
Review-Url: https://codereview.webrtc.org/2887593003
Cr-Commit-Position: refs/heads/master@{#19656}