Update ButtonBarViewCell.swift

Add UIKit import as it creates a compile time error

Author: Khronoss

Sources/ButtonBarViewCell.swift

@@ -22,7 +22,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
-import Foundation
+import UIKit
 
 open class ButtonBarViewCell: UICollectionViewCell {