nir: fix unused variable warning in nir_lower_vars_to_explicit_types

nir: fix unused variable warning in nir_lower_vars_to_explicit_types

This commit fixes the following warning:
../src/compiler/nir/nir_lower_io.c: In function ‘nir_lower_vars_to_explicit_types’:
../src/compiler/nir/nir_lower_io.c:1435:22: warning: unused variable ‘supported’ [-Wunused-variable]
 1435 |    nir_variable_mode supported = nir_var_mem_shared | nir_var_shader_temp | nir_var_function_temp;
      |                      ^~~~~~~~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>