Give CGAL a boost::prior (import) (#3978)

CGAL uses boost::prior but doesn't import it properly. A small hack here
just imports it before importing anything CGAL.

src/libslic3r/Geometry/VoronoiUtilsCgal.cpp

@@ -1,3 +1,4 @@
+#include <boost/next_prior.hpp>
 #include <CGAL/Exact_predicates_exact_constructions_kernel.h>
 #include <CGAL/Arr_segment_traits_2.h>
 #include <CGAL/Surface_sweep_2_algorithms.h>
@@ -100,4 +101,4 @@ bool VoronoiUtilsCgal::is_voronoi_diagram_planar_angle(const VoronoiDiagram &vor
 }
 
 
-} // namespace Slic3r::Geometry
+} // namespace Slic3r::Geometry