Correct a macro

--enable-lowbitdepth defines the flag CONFIG_LOWBITDEPTH, not
CONFIG_AOM_LOWBITDEPTH.

Change-Id: Ifa1c12847bee4978d08d010f4fc3601d75e59c31
diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 57849cb..24451db 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -3972,7 +3972,7 @@
   if (cm->bit_depth > AOM_BITS_8) {
     cm->use_highbitdepth = 1;
   } else {
-#if CONFIG_AOM_LOWBITDEPTH
+#if CONFIG_LOWBITDEPTH
     cm->use_highbitdepth = 0;
 #else
     cm->use_highbitdepth = 1;