Add include guards to CMake files used as includes.

BUG=aomedia:76

Change-Id: Ie34025f31a89f4991d03d5ecf03c6f6f5ab7b0a1
diff --git a/aom_util/aom_util.cmake b/aom_util/aom_util.cmake
index 484d9d9..68f5253 100644
--- a/aom_util/aom_util.cmake
+++ b/aom_util/aom_util.cmake
@@ -8,6 +8,9 @@
 ## Media Patent License 1.0 was not distributed with this source code in the
 ## PATENTS file, you can obtain it at www.aomedia.org/license/patent.
 ##
+if (NOT AOM_AOM_UTIL_AOM_UTIL_CMAKE_)
+set(AOM_AOM_UTIL_AOM_UTIL_CMAKE_ 1)
+
 set(AOM_UTIL_SOURCES
     "${AOM_ROOT}/aom_util/aom_thread.c"
     "${AOM_ROOT}/aom_util/aom_thread.h"
@@ -27,3 +30,5 @@
   set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_util PARENT_SCOPE)
   target_sources(aom PUBLIC $<TARGET_OBJECTS:aom_util>)
 endfunction ()
+
+endif ()  # AOM_AOM_UTIL_AOM_UTIL_CMAKE_