remove gcc noexcept warning

[beinhaerter]

Without this change a gsl::not_null<class_type> triggers these noexcept warnings:

.../gsl/include/gsl/pointers:162:50: warning: noexcept-expression evaluates to ‘false’ because of a call to ‘constexpr gsl::details::value_or_reference_return_t<T> gsl::not_null<T>::get() const [with T = class_type*; gsl::details::value_or_reference_return_t<T> = class_type* const]’ [-Wnoexcept]
  162 |                 const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
      |                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../gsl/include/gsl/pointers:119:55: note: but ‘constexpr gsl::details::value_or_reference_return_t<T> gsl::not_null<T>::get() const [with T = class_type*; gsl::details::value_or_reference_return_t<T> = class_type* const]’ does not throw; perhaps it should be declared ‘noexcept’
  119 |     constexpr details::value_or_reference_return_t<T> get() const
      |                                                       ^~~

[hsutter]

Editors' call: Looks good, thanks!