only export ment_cmake_core instead of ament_cmake (#459)

Signed-off-by: Dirk Thomas <[email protected]>

Author: dirk-thomas

rosidl_generator_c/package.xml

@@ -10,7 +10,7 @@
   <buildtool_depend>ament_cmake_python</buildtool_depend>
   <buildtool_depend>ament_cmake_ros</buildtool_depend>
 
-  <buildtool_export_depend>ament_cmake</buildtool_export_depend>
+  <buildtool_export_depend>ament_cmake_core</buildtool_export_depend>
   <buildtool_export_depend>rosidl_cmake</buildtool_export_depend>
 
   <build_export_depend>rosidl_typesupport_interface</build_export_depend>

rosidl_generator_cpp/package.xml

@@ -10,7 +10,7 @@
 
   <buildtool_depend>ament_cmake</buildtool_depend>
 
-  <buildtool_export_depend>ament_cmake</buildtool_export_depend>
+  <buildtool_export_depend>ament_cmake_core</buildtool_export_depend>
   <buildtool_export_depend>rosidl_cmake</buildtool_export_depend>
 
   <!-- This is needed for the rosidl_message_type_support_t struct and visibility macros -->