Add vp9_highbitdepth info in configure --help

Change-Id: I89497a483117fc472be3a836070109498258140d

configure

@@ -58,6 +58,7 @@ Advanced options:
   ${toggle_postproc_visualizer}   macro block / block level visualizers
   ${toggle_multi_res_encoding}    enable multiple-resolution encoding
   ${toggle_temporal_denoising}    enable temporal denoising and disable the spatial denoiser
+  ${toggle_vp9_highbitdepth}      enable 10/12 bit support in VP9
   ${toggle_vp9_temporal_denoising}
                                   enable vp9 temporal denoising
   ${toggle_webm_io}               enable input from and output to WebM container