Fix missing header in ResourceUsage.h

[SF-N]

ResourceUsage.h used size_t, but did not include the stddef header. This lead to some compilation errors in atlas4py.

[FussyDuck]

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[pmaciel]

Note that the #include above takes care of this

[SF-N]

It wasn't working with clang-1500.3.9.4 and when looking at https://en.cppreference.com/w/c/types/size_t it seems like the includes above don't take care of this.

[wdeconinck]

Something strange, because you should not use the C compiler but rather the C++ compiler, and there size_t should be defined in many possible headers including cstddef as is already included.
"https://en.cppreference.com/w/cpp/types/size_t"

Please could you provide us the platform information, full compiler invocation and error message?

[SF-N]

@wdeconinck and I just figured it out: it was due to an older eckit-version used in atlas4py which didn't include that header.