Add missing function prototype.

Firmware/Marlin_main.cpp

@@ -407,6 +407,7 @@ static void get_arc_coordinates();
 static bool setTargetedHotend(int code, uint8_t &extruder);
 static void print_time_remaining_init();
 static void wait_for_heater(long codenum, uint8_t extruder);
+static void gcode_G28(bool home_x_axis, bool home_y_axis, bool home_z_axis);
 
 uint16_t gcode_in_progress = 0;
 uint16_t mcode_in_progress = 0;