Add rvalue qualified xstrided_view_base::storage

[AntoinePrv]

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Description

Fixes #2401

[AntoinePrv]

@JohanMabille , I haven't been able to figure out this build error:

.../include/xtensor/xstrided_view_base.hpp:316:11: error: rvalue reference to type
'xt::xstrided_view_base<... >::storage_type' (aka 'xt::uvector<int, std::allocator<int> >')
cannot bind to lvalue of type 'xt::uvector<int, std::allocator<int> >'

I (sort of) understand rvalue references cannot bind to lvalue, but I don't understand why this is happening.