Fix "Undefined symbols" linker error when DocumentChange::npos was used by dconeybe · Pull Request #474

Fix "Undefined symbols" linker error when DocumentChange::npos was used by dconeybe · Pull Request #474 · firebase/firebase-cpp-sdk

Fix a bug where accessing firebase::firestore::DocumentChange::npos from a non-Android platform results in a linker error:

Undefined symbols for architecture x86_64:
  "firebase::firestore::DocumentChange::npos", referenced from:
      firebase_testapp_automated::FirebaseFirestoreBasicTest_TestDocumentChangeNpos_Test::TestBody() in integration_test.cc.o
ld: symbol(s) not found for architecture x86_64

Here is the declaration of npos:

	#if defined(ANDROID)
	// Older NDK (r16b) fails to define this properly. Fix this when support for
	// the older NDK is removed.
	static const std::size_t npos;
	#else
	static constexpr std::size_t npos = static_cast<std::size_t>(-1);
	#endif // defined(ANDROID)

The fix is to add a definition of npos into the corresponding .cc file