dda.c: add forgotten variable declaration.

Forgotten in commit 74808610c7,
"DDA: Move axis calculations into loops, part 5.".

This and the previous commit makes ACCELERATION_TEMPORAL building
(and working!) again.

dda.c

@@ -682,6 +682,7 @@ void dda_step(DDA *dda) {
       short-delays consecutively and to give sufficient time on average.
    */
 		uint32_t c_candidate;
+    uint8_t i;
 
 		dda->c = 0xFFFFFFFF;
     for (i = X; i < AXIS_COUNT; i++) {