add import to fix builds failing on iOS (#512)

fixes https://github.com/henninghall/react-native-date-picker/issues/506

ios/RNDatePicker/DatePicker.m

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */
-
+#import "RCTComponent.h"
 #import "DatePicker.h"
 
 #import "RCTUtils.h"