fix namespace include in cmakelists for mlspp, backport of mls++ fix

[braindigitalis]

backport from MLS++ as requested by @egorpugin

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
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