Fix xcode build errors re nanopb
Fix xcode build errors re nanopb

Involves adding PODS_ROOT/nanopb to include path (to allow include
<pb.h>) and Firestore/Protos/nanopb to include path (to allow include
"google/api/annotations.pb.h" and similar).

In both cases, this is to allow auto-generated code to function
properly.

firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk