remove gcc noexcept warning (#1122)
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
| ^~~
```
Co-authored-by: Werner Henze <w.henze@avm.de>