Add missing gRPC include.

Othewise the header is not self contained and breaks when
upgrading gRPC.

Bug: 279548314
Test: presubmit
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:3d3baed1bc85f8ddddb2764a5a09cd4d578419f0)
Merged-In: Ife50863bd16eda6bc639261b1f91fdfcf48f6294
Change-Id: Ife50863bd16eda6bc639261b1f91fdfcf48f6294