fix ACCELERATION_REPRAP build (moved/extra vars)

REPRAP style acceleration broke quite a while ago, but no one noticed.
Maybe it's not being used, and therefore also not tested. But it should
at least compile while it remains an option.

The compiler complains that dda->n is not defined and that current_id is
never used.  The first bug goes back to f0b9daeea0 in late 2013.

In the interest of supporting exploratory accelerations, fix this to
build when ACCELERATION_REPRAP is chosen.

dda.c

@@ -756,8 +756,8 @@ void dda_clock() {
   uint32_t move_step_no, move_c;
   int32_t move_n;
   uint8_t recalc_speed;
-  #endif
   uint8_t current_id ;
+  #endif
 
   dda = queue_current_movement();
   if (dda != last_dda) {

dda.h

@@ -119,6 +119,7 @@ typedef struct {
 
 	#ifdef ACCELERATION_REPRAP
 	uint32_t					end_c; ///< time between 2nd last step and last step
+	int32_t						n;     ///< precalculated step time offset variable
 	#endif
 	#ifdef ACCELERATION_RAMPING
   /// precalculated step time offset variable