cmake: Use list(APPEND) instead of set() in aom_util.cmake

Change-Id: I6531675f2195de7108702f7c2ad4ae04d131ad71
diff --git a/aom_util/aom_util.cmake b/aom_util/aom_util.cmake
index c449059..d4f3bce 100644
--- a/aom_util/aom_util.cmake
+++ b/aom_util/aom_util.cmake
@@ -13,9 +13,11 @@
 endif() # 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" "${AOM_ROOT}/aom_util/endian_inl.h"
-    "${AOM_ROOT}/aom_util/debug_util.c" "${AOM_ROOT}/aom_util/debug_util.h")
+list(APPEND AOM_UTIL_SOURCES "${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")
 
 # Creates the aom_util build target and makes libaom depend on it. The libaom
 # target must exist before this function is called.