Fix signed / unsigned comparison warning from meta_enum.h
Fixes a minor warning exposed by d0 builds
Kernel/LHCbKernel/Kernel/meta_enum.h:204:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::array<meta_enum_internal::MetaEnumMember<MyEnum2>, 5>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
not seen in other builds as (presumably) code is constexpr optimised away, and the compiler does not complain in this case.