graphics/mapnik: unbreak with boost 1.69

In file included from src/save_map.cpp:40:
include/mapnik/image_filter.hpp:34:10: fatal error: 'boost/gil/gil_all.hpp' file not found
#include <boost/gil/gil_all.hpp>

^~~~~~~~~~~~~~~~~~~~~~~

deps/agg/src/agg_pixfmt_rgba.cpp:6:10: fatal error: 'boost/gil/gil_all.hpp' file not found
#include <boost/gil/gil_all.hpp>

^~~~~~~~~~~~~~~~~~~~~~~

PR: 232525