Adds export for link_reference_definition_syntax.dart (dart-lang/markdown#626)

pkgs/markdown/lib/markdown.dart

diff --git a/pkgs/markdown/lib/markdown.dart b/pkgs/markdown/lib/markdown.dart
index a09e763..9fac732 100644
--- a/pkgs/markdown/lib/markdown.dart
+++ b/pkgs/markdown/lib/markdown.dart
@@ -55,6 +55,7 @@
 export 'src/block_syntaxes/header_with_id_syntax.dart';
 export 'src/block_syntaxes/horizontal_rule_syntax.dart';
 export 'src/block_syntaxes/html_block_syntax.dart';
+export 'src/block_syntaxes/link_reference_definition_syntax.dart';
 export 'src/block_syntaxes/list_syntax.dart';
 export 'src/block_syntaxes/ordered_list_syntax.dart';
 export 'src/block_syntaxes/ordered_list_with_checkbox_syntax.dart';