Address Firestore review feedback

Author: paulb777

Firestore/Source/API/FIRFirestoreVersion.mm

@@ -22,4 +22,4 @@
 
 // Because `kFirestoreVersionString` is subject to constant initialization, this
 // is not affected by static initialization order fiasco.
-extern "C" const char *const FIRFirestoreVersionString = (const char *const)kFirestoreVersionString;
+extern "C" const char *const FIRFirestoreVersionString = kFirestoreVersionString;

Firestore/core/test/firebase/firestore/auth/firebase_credentials_provider_test.mm

@@ -17,6 +17,7 @@
 #include "Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h"
 
 #import <FirebaseAuthInterop/FIRAuthInterop.h>
+#import <FirebaseCore/FIRApp.h>
 
 #include <chrono>  // NOLINT(build/c++11)
 #include <future>  // NOLINT(build/c++11)