Fix gcc11 annonymous namespace warnings

Roel Aaijrequested to merge
raaij/Gaudi:inline_histogram_naming into master

This fixes warnings of the following type:

warning: 'MEPProvider' has a field 'MEPProvider::m_bad_mfps_sd' whose type uses the anonymous namespace [-Wsubobject-linkage]

where MEPProvider is a Service and m_bad_mfps_sd is a Gaudi::Accumulators::Histogram<1, Gaudi::Accumulators::atomicity::full, float>.

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