more missing includes (#6398)

Here are more missing header files recently identified by building
without precompiled header support.

src/libslic3r/Geometry/MedialAxis.cpp

@@ -1,7 +1,9 @@
+#include <boost/log/trivial.hpp>
 #include "MedialAxis.hpp"
 
 #include "clipper.hpp"
 #include "VoronoiOffset.hpp"
+#include "../ClipperUtils.hpp"
 
 #ifdef SLIC3R_DEBUG
 namespace boost { namespace polygon {

src/libslic3r/Geometry/VoronoiUtilsCgal.cpp

@@ -1,3 +1,5 @@
+// Needed since the CGAL headers are not self-contained.
+#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>