Fix Storage private imports. (#2206)

Author: ryanwilson

Firebase/Storage/Private/FIRStorageDownloadTask_Private.h

@@ -14,6 +14,11 @@
  * limitations under the License.
  */
 
+#import <Foundation/Foundation.h>
+
+#import "FIRStorageDownloadTask.h"
+
+@class FIRStorageReference;
 @class GTMSessionFetcherService;
 
 NS_ASSUME_NONNULL_BEGIN

Firebase/Storage/Private/FIRStorageObservableTask_Private.h

@@ -14,10 +14,14 @@
  * limitations under the License.
  */
 
+#import <Foundation/Foundation.h>
+
+#import "FIRStorageObservableTask.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
+@class FIRStorageReference;
 @class FIRStorageTaskSnapshot;
-
 @class GTMSessionFetcherService;
 
 @interface FIRStorageObservableTask ()

Firebase/Storage/Private/FIRStorageTaskSnapshot_Private.h

@@ -14,6 +14,10 @@
  * limitations under the License.
  */
 
+#import <Foundation/Foundation.h>
+
+#import "FIRStorageTaskSnapshot.h"
+
 #import "FIRStorageConstants_Private.h"
 
 NS_ASSUME_NONNULL_BEGIN