bitstream_debug: build related cleanup

Move experimental config from debug_util.c/h to aom_util.mk to avoid
empty object.

Change-Id: Id7978ed6a342262bddaa4df8b53115e750fa1c2c
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index ce16fcf..4f50302 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -19,7 +19,9 @@
 #include "aom_mem/aom_mem.h"
 #include "aom_ports/mem_ops.h"
 #include "aom_ports/system_state.h"
+#if CONFIG_BITSTREAM_DEBUG
 #include "aom_util/debug_util.h"
+#endif  // CONFIG_BITSTREAM_DEBUG
 
 #if CONFIG_CLPF
 #include "av1/common/clpf.h"