AP_DDS: Added namespace representation for global constants

[snktshrma]

This updates global constant in sync with this PR: https://github.com/ArduPilot/Micro-XRCE-DDS-Gen/pull/5

[TannerGilbert]

Without this PR building Ardupilot Gazebo SITL currently fails with:

[‘FS_RADIO’ was not declared in this scope; did you mean ‘FLT_RADIX’](https://discuss.ardupilot.org/t/dds-sitl-fs-radio-was-not-declared-in-this-scope-did-you-mean-flt-radix/134537)

Related discussion: https://discuss.ardupilot.org/t/dds-sitl-fs-radio-was-not-declared-in-this-scope-did-you-mean-flt-radix/134537/3

[zhouzhiwen2000]

Without this PR building Ardupilot Gazebo SITL currently fails with:

[‘FS_RADIO’ was not declared in this scope; did you mean ‘FLT_RADIX’](https://discuss.ardupilot.org/t/dds-sitl-fs-radio-was-not-declared-in-this-scope-did-you-mean-flt-radix/134537)

Related discussion: https://discuss.ardupilot.org/t/dds-sitl-fs-radio-was-not-declared-in-this-scope-did-you-mean-flt-radix/134537/3

Can confirm this PR works.

[peterbarker]

.... rebased it in the hopes that CI passes

[peterbarker]

Dies due to the tool being old, I think?

[snktshrma]

Dies due to the tool being old, I think?

Yes seems like still the CI autotests not using latest Micro-XRCE-DDS-Gen

[peterbarker]

I've pushed up another patch here to install the generator from source, which should get CI to pass.