vulkan: fix compilation on 32-bit

VkDeviceSize != size_t on 32-bit platforms, which makes this a pointer
type mismatch.

Fixes https://github.com/haasn/libplacebo/issues/95
12 jobs for master in 9 minutes and 46 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Compile
passed aarch64 #595900
docker aarch64

00:00:09

passed linux #595896
docker amd64

00:00:10

passed macos #595901
amd64 catalina

00:00:08

passed scan #595902
docker amd64

00:00:26

passed static #595897
docker amd64

00:00:10

passed win32 #595898
docker amd64

00:00:10

passed win64 #595899
amd64 docker

00:00:11

 
  Test
passed gpu #595904
gpu

00:03:24

78.8285%
passed llvmpipe #595903
docker amd64

00:00:27

 
  Sanitize
passed asan #595905
gpu

00:02:16

passed msan #595906
gpu

00:01:59

passed ubsan #595907
gpu

00:01:36