CMakeLists.txt: add -DPB_NO_PACKED_STRUCTS=1 to the nanopb compiler flags

[dconeybe]

This fixes a flurry of "ld: warning: pointer not aligned at address" warnings that reportedly fail the build of the C++ SDK.

Here are some customer reports of these warnings affecting builds:

• [Bug] Firestore fails to link for desktop on macOS Monterey because of unaligned pointers firebase-cpp-sdk#712
• Firestore SDK doesn't build on Apple Silicon (pointer misalignment during linking) firebase-unity-sdk#83

[google-oss-bot]

Coverage Report

Affected SDKs

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  SDK overall coverage changed from 87.70% (04b7555) to 87.68% (8aedfa2) by -0.02%.

  Filename	Base (04b7555)	Head (8aedfa2)	Diff
  leveldb_key.cc	96.75%	97.37%	+0.62%
  ordered_code.cc	92.18%	91.16%	-1.02%
  serializer.cc	87.65%	87.11%	-0.54%
  task.cc	93.04%	94.78%	+1.74%
  watch_stream.cc	95.35%	90.70%	-4.65%

Test Logs

• Base (04b75552): https://github.com/firebase/firebase-ios-sdk/actions/runs/1415015377
• Head (8aedfa23): https://github.com/firebase/firebase-ios-sdk/actions/runs/1415389911