Avoid compiling error

Only a missing ","... now it will compile again

Marlin/Marlin_main.cpp

@@ -254,6 +254,7 @@ float extruder_offset[NUM_EXTRUDER_OFFSETS][EXTRUDERS] = {
   #else
     0
   #endif
+  ,
   #if defined(EXTRUDER_OFFSET_Y)
     EXTRUDER_OFFSET_Y
   #else