Fix lcd_babystep_zoffset()

Resolve 'steps_to_mm was not declared in this scope' compilation error after recent changes to merge babystepping with M851

Marlin/ultralcd.cpp

           const int babystep_increment = (int32_t)encoderPosition * (BABYSTEP_MULTIPLICATOR);
           encoderPosition = 0;
 
-          const float new_zoffset = zprobe_zoffset + steps_to_mm[Z_AXIS] * babystep_increment;
+          const float new_zoffset = zprobe_zoffset + planner.steps_to_mm[Z_AXIS] * babystep_increment;
           if (WITHIN(new_zoffset, Z_PROBE_OFFSET_RANGE_MIN, Z_PROBE_OFFSET_RANGE_MAX)) {
 
             if (planner.abl_enabled)