Advance the request buffer by the right amount in XIChangeHierarchy

c->length is in 4-byte units, dptr is a char *, so we need to advance
dptr by 4 * length to get the position of the next HierarchyChangeInfo.

Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Julien Cristau <jcristau@debian.org>