add rosidl_generator_dds_idl dependency to package.xml

[kbrameld]

This allows rosidl_generator_dds_idl to be installed with rosdep, the common way to install dependencies. Without it, users may encounter this error

--- stderr: unitree_api
CMake Error at CMakeLists.txt:26 (find_package):
  By not providing "Findrosidl_generator_dds_idl.cmake" in CMAKE_MODULE_PATH
  this project has asked CMake to find a package configuration file provided
  by "rosidl_generator_dds_idl", but CMake did not find one.
  Could not find a package configuration file provided by
  "rosidl_generator_dds_idl" with any of the following names:
    rosidl_generator_dds_idlConfig.cmake
    rosidl_generator_dds_idl-config.cmake
  Add the installation prefix of "rosidl_generator_dds_idl" to
  CMAKE_PREFIX_PATH or set "rosidl_generator_dds_idl_DIR" to a directory
  containing one of the above files.  If "rosidl_generator_dds_idl" provides
  a separate development package or SDK, be sure it has been installed.
---
Failed   <<< unitree_api [1.39s, exited with code 1]