st/mesa: silence missing braces warning in st_program.c

st/mesa: silence missing braces warning in st_program.c

Silence a gcc warning:
state_tracker/st_program.c: In function 'st_create_fp_variant':
state_tracker/st_program.c:957:10: warning: missing braces around initializer [-Wmissing-braces]
          nir_lower_drawpixels_options options = {0};
          ^
state_tracker/st_program.c:957:10: warning: (near initialization for 'options.texcoord_state_tokens') [-Wmissing-braces]

Reviewed-by: Marek Olšák <marek.olsak@amd.com>