pvr: Force compile error on GNU void pointer arithmetic

pvr: Force compile error on GNU void pointer arithmetic

This GNU extension caused a bug where a void pointer was modified then
interpreted as uint32_t* (instead of the other way round). Force a
compile error to (hopefully) prevent this from happening again.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25327>