Change experimental flag name

CONFIG_UV_LVL --> CONFIG_LOOPFILTER_LEVEL
Change name because the experiment combines filter level search
in U, V plane, and horizontal, vertical search in Y plane.

Change-Id: Ia05b8f80cd93b60a0305eb94e5783559ef904c13
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 4663e82..1ab2512 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -3419,7 +3419,7 @@
   struct loopfilter *lf = &cm->lf;
 
 // Encode the loop filter level and type
-#if CONFIG_UV_LVL
+#if CONFIG_LOOPFILTER_LEVEL
   aom_wb_write_literal(wb, lf->filter_level[0], 6);
   aom_wb_write_literal(wb, lf->filter_level[1], 6);
   if (lf->filter_level[0] || lf->filter_level[1]) {