Add missing export build dependency

This dependency was accidentally removed in #49.
We have a dependency on a specific version since required behvavior changed in #41.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rosidl_typesupport_connext_c/package.xml

@@ -17,6 +17,7 @@
  <buildtool_export_depend>connext_cmake_module</buildtool_export_depend>
  <buildtool_export_depend>rcutils</buildtool_export_depend>
  <buildtool_export_depend>rosidl_cmake</buildtool_export_depend>
+ <buildtool_export_depend version_gte="0.9.0">rosidl_generator_c</buildtool_export_depend>
  <buildtool_export_depend>rosidl_runtime_c</buildtool_export_depend>
  <buildtool_export_depend>rosidl_generator_dds_idl</buildtool_export_depend>
  <buildtool_export_depend>rosidl_typesupport_connext_cpp</buildtool_export_depend>