Homogenize configuration option name.

Rename '--enable-aom-highbitdepth' to '--enable-highbitdepth'

Change-Id: I1de13c3508c30c552532993419d8ace326142ab6
diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index 84fe8e2..6d3497f 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -258,7 +258,7 @@
 
   aom_tune_metric tuning;
   aom_tune_content content;
-#if CONFIG_AOM_HIGHBITDEPTH
+#if CONFIG_HIGHBITDEPTH
   int use_highbitdepth;
 #endif
   aom_color_space_t color_space;