seat/pointer: fix uninitialized variable warning

This results in the following warning, which in release mode causes an
error due to -Werror:

../types/seat/wlr_seat_pointer.c: In function ‘wlr_seat_pointer_send_axis’:
../types/seat/wlr_seat_pointer.c:344:25: error: ‘low_res_value_discrete’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  343 |   if (version < WL_POINTER_AXIS_VALUE120_SINCE_VERSION &&
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  344 |     value_discrete != 0 && low_res_value_discrete == 0) {
      |     ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

types/seat/wlr_seat_pointer.c

@@ -328,7 +328,7 @@ void wlr_seat_pointer_send_axis(struct wlr_seat *wlr_seat, uint32_t time,
 	}
 
 	double low_res_value;
-	int32_t low_res_value_discrete;
+	int32_t low_res_value_discrete = 0;
 	update_value120_accumulators(client, orientation, value, value_discrete,
 		&low_res_value, &low_res_value_discrete);