Update CMakeLists.txt

Author: schmidt-sebastian

firestore/integration_test_internal/CMakeLists.txt

@@ -206,6 +206,7 @@ include_directories(${FIREBASE_CPP_SDK_DIR})
 # Additional public headers from Firestore core SDK.
 include_directories(${PROJECT_BINARY_DIR}/bin/external/src/firestore)
 include_directories(${PROJECT_BINARY_DIR}/bin/external/src/nanopb)
+include_directories(${PROJECT_BINARY_DIR}/bin/external/src/firestore/Firestore/Protos/nanopb)
 # Additional public headers for absl.
 include_directories(${ABSEIL_CPP_ROOT})
 # Allow testing internal Firebase APIs.