Missing Foundation import (#6670)

Author: paulb777

FirebaseABTesting/Sources/ABTConstants.h

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#import <Foundation/Foundation.h>
 #import "FirebaseCore/Sources/Private/FIRLogger.h"
 
 #define ABT_MSEC_PER_SEC 1000ull