Input: fix sparse warning in multitouch code

This fixes the following sparse warning:
drivers/input/input-mt.c:193:18: warning: Using plain integer as NULL pointer

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/input-mt.c

@@ -190,7 +190,7 @@ void input_mt_report_pointer_emulation(struct input_dev *dev, bool use_count)
 	if (!mt)
 		return;
 
-	oldest = 0;
+	oldest = NULL;
 	oldid = mt->trkid;
 	count = 0;