[FIX] compilation error due to isspace() in usddeck.c

[jungr-ait]

HINT: resolves error: array subscript has type 'char'
[-Werror=char-subscripts] as all warnings are treated as errors.
This error occurs when running the build.sh script from the crazyswarm (master branch) project.

[whoenig]

Hi,
We try to keep the crazyswarm firmware as close as possible to the official firmware. This issue exists in the official firmware and can be fixed by upgrading your compiler (see bitcraze#418). If you want to change the code anyways, I suggest that you re-open the issue in the official firmware and create a PR there. We will pick up the change for the crazyswarm during our next merge.
Thanks!

[jungr-ait]

Thank you for your fast reply and the link to the issue,

I followed to official instructions and I am working with a docker container. I cannot build the container due to this error. So either we need to fix this bug or change the official documentation in case of Ubuntu 16.04 (https://crazyswarm.readthedocs.io/en/latest/installation.html).