FIX: Use the same api to get the extruder id

jira: none
Change-Id: I05b3040b176374deee3e95bc52364fe7b33bb257
(cherry picked from commit 8b2544df41456377b09719de15460b19b7c71e09)

src/libslic3r/Extruder.cpp

@@ -18,6 +18,11 @@ Extruder::Extruder(unsigned int id, GCodeConfig *config, bool share_extruder) :
     m_e_per_mm3 /= this->filament_crossection();
 }
 
+unsigned int Extruder::extruder_id() const
+{
+    return get_extruder_index(m_id);
+}
+
 double Extruder::extrude(double dE)
 {
     // BBS