Fix -Wignored-reference-qualifiers

[georgthegreat]

In certain STLs, std::vector<bool>::reference is just bool&.
Prepending const causes the following warning:

include/nlohmann/detail/conversions/to_json.hpp:271:42: error: 'const' qualifier on reference type 'std::vector<bool>::reference' (aka 'bool &') has no effect [-Werror,-Wignored-reference-qualifiers]

This PR fixes the problem.

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[falbrechtskirchinger]

Which STL would that be? ( So we can test.)

[georgthegreat]

We use custom STL with std::vector compation disabled.
This is the vector we use.

[falbrechtskirchinger]

I see.

I did fix it slightly differently (#3678). We can simply disable the to_json function in that case as we already have one for bool.
That should work for you, right?