add missing export r_sizedbox.dart

@@ -6,6 +6,7 @@
 library flutter_screenutil;
 
 export 'src/r_padding.dart';
+export 'src/r_sizedbox.dart';
 export 'src/screen_util.dart';
 export 'src/screenutil_init.dart';
 export 'src/size_extension.dart';