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>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>