FIX: backend get the extruder id based on filament_map

Change-Id: Ib7679c0fc67336e462467dab9f5b4d4684d6eb19
(cherry picked from commit dcd9fd501354da33baea2adc0f645fabe8880cf1)

src/libslic3r/Layer.hpp

@@ -241,6 +241,8 @@ public:
         return idx;
     }
 
+    size_t get_extruder_id(unsigned int filament_id) const;
+
 protected:
     friend class PrintObject;
     friend std::vector<Layer*> new_layers(PrintObject*, const std::vector<coordf_t>&);