Fix build error when PCH is not used

src/libslic3r/Feature/FuzzySkin/FuzzySkin.cpp

@@ -3,6 +3,7 @@
 #include "libslic3r/Algorithm/LineSplit.hpp"
 #include "libslic3r/Arachne/utils/ExtrusionJunction.hpp"
 #include "libslic3r/Arachne/utils/ExtrusionLine.hpp"
+#include "libslic3r/ClipperUtils.hpp"
 #include "libslic3r/Layer.hpp"
 #include "libslic3r/PerimeterGenerator.hpp"
 #include "libslic3r/Point.hpp"

src/libslic3r/Feature/Interlocking/InterlockingGenerator.cpp

@@ -2,6 +2,7 @@
 // CuraEngine is released under the terms of the AGPLv3 or higher.
 
 #include "InterlockingGenerator.hpp"
+#include "libslic3r/ClipperUtils.hpp"
 #include "Layer.hpp"
 
 namespace std {