Make locator.hpp self contained

[sdebionne]

Description

locator implementation uses make_step_iterator() but step_iterator.hpp is not included.

Tasklist

• [x] Ensure all CI builds pass
• [x] Review and approve

[sdebionne]

This fixes the following issue:

boost/gil/locator.hpp:258:32: error: 'make_step_iterator' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
  258 |         : _p(make_step_iterator(loc.x(),(transpose ? loc.row_size() : loc.pixel_size())*x_step),
      |              ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...
note: 'template<class I> typename boost::gil::dynamic_x_step_type<T>::type boost::gil::make_step_iterator(const I&, std::ptrdiff_t)' declared here, later in the translation unit

[sdebionne]

@mloskot Do you think this little change could make it to master for 1.82?

[mloskot]

@sdebionne I see no reason why not, unless the release managers object it.