st/nir: use asprintf() wrapper to fix MSVC issues

Fixes: 856e8408 ("mesa/st: add sampler uniforms")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
13 jobs for !1395 with msvc-asprintf in 11 minutes and 17 seconds (queued for 35 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #439950
debian

00:00:24

 
  Build+Test
passed #439959
meson-arm64

00:02:35

passed #439958
meson-armhf

00:02:08

passed #439952
meson-clang

00:09:06

passed #439956
meson-clover

00:09:46

passed #439960
meson-i386

00:02:38

passed #439955
meson-main

00:06:06

passed #439951
meson-swr-glvnd

00:06:28

passed #439957
meson-vulkan

00:03:10

passed #439962
scons-llvm

00:03:21

passed #439961
scons-nollvm

00:03:00

passed #439953
scons-swr

00:06:09

passed #439954
scons-win64

00:07:00