nir: fix unused variable warning in find_and_update_previous_uniform_storage

nir: fix unused variable warning in find_and_update_previous_uniform_storage

This commit fixes the following warning:
../src/compiler/glsl/gl_nir_link_uniforms.c: In function ‘find_and_update_previous_uniform_storage’:
../src/compiler/glsl/gl_nir_link_uniforms.c:166:16: warning: unused variable ‘num_blks’ [-Wunused-variable]
  166 |       unsigned num_blks = nir_variable_is_in_ubo(var) ?
      |                ^~~~~~~~

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