xinput: XIGetProperty-reply: add align-pad at end

reasons:
* all X11-replies must have a length which is a multiple of 4 bytes
* bitcases "8Bits" and "16bits" may have a length
  which is not a multiple of 4 bytes
  but they start on a 4-byte aligned position.
  Therefore the unpadded end of the request may not be 4-byte aligned
* Therefore this requires a 4-byte align-pad.

notes:
* The align pad had to be added in each relevant bitcase
  because adding it after the switch would cause the generator
  to abort. (which is OK because a <switch> has to be the last item
  of a struct, request or reply, according to the xcb-xml-spec)