-Wdeprecated-literal-operator from clang++-20 for 'operator"" _c' (i.e. with space)

[MichaelChirico]

histogram/include/boost/histogram/literals.hpp

Line 26 in 6684266

auto operator"" _c() {

The fix can be trivial, but I think there's some back-compatibility considerations I lack the context to adjudicate, see sister issues e.g.

boostorg/hana#521