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 {