Switch to #import <Foundation/Foundation.h> in FIRAppCheckProtocol.h (#11864)

Author: andrewheard

FirebaseAppCheck/Interop/FIRAppCheckProtocol.h

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-@import Foundation;
+#import <Foundation/Foundation.h>
 
 @class FIRAppCheckToken;