aom_util.cmake: conditionally add debug_util.[ch]

These files are only needed with CONFIG_BITSTREAM_DEBUG.

Bug: aomedia:3416
Change-Id: If69e71210339856b3e820ea8eb51ebd523e86615
diff --git a/aom_util/aom_util.cmake b/aom_util/aom_util.cmake
index 1a1bfe1..6bf4faf 100644
--- a/aom_util/aom_util.cmake
+++ b/aom_util/aom_util.cmake
@@ -15,9 +15,12 @@
 
 list(APPEND AOM_UTIL_SOURCES "${AOM_ROOT}/aom_util/aom_thread.c"
             "${AOM_ROOT}/aom_util/aom_thread.h"
-            "${AOM_ROOT}/aom_util/endian_inl.h"
-            "${AOM_ROOT}/aom_util/debug_util.c"
-            "${AOM_ROOT}/aom_util/debug_util.h")
+            "${AOM_ROOT}/aom_util/endian_inl.h")
+
+if(CONFIG_BITSTREAM_DEBUG)
+  list(APPEND AOM_UTIL_SOURCES "${AOM_ROOT}/aom_util/debug_util.c"
+              "${AOM_ROOT}/aom_util/debug_util.h")
+endif()
 
 # Creates the aom_util build target and makes libaom depend on it. The libaom
 # target must exist before this function is called.