nir: use braces around subobject in initializer

Used same syntax as elsewhere with Mesa sources, verified result
against MSVC with godbolt.org.

fixes following warning with clang:
   warning: suggest braces around initialization of subobject

v2: empty braces -> braces around subobject (Caio, Kristian)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
14 jobs for !717 with use-empty-braces in 10 minutes and 32 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #261551
debian

00:00:25

 
  Build+Test
passed #261553
meson-clang

00:10:06

passed #261559
meson-gallium-clover-llvm

00:09:09

passed #261560
meson-gallium-clover-llvm39

00:05:50

passed #261558
meson-gallium-drivers-other

00:07:20

passed #261557
meson-gallium-radeonsi

00:04:28

passed #261561
meson-gallium-st-other

00:05:56

passed #261552
meson-gallium-swr

00:08:55

passed #261556
meson-glvnd

00:04:47

passed #261555
meson-loader-classic-dri

00:06:15

passed #261554
meson-vulkan

00:03:36

passed #261563
scons-llvm

00:04:59

passed #261562
scons-nollvm

00:04:40

passed #261564
scons-swr

00:06:50