aom_mem.cmake: Use list(APPEND) instead of set().

Change-Id: Ia7df73b5cd8f242228bc4f8dacff8212600fa7ff
diff --git a/aom_mem/aom_mem.cmake b/aom_mem/aom_mem.cmake
index d4bd38a..eaee844 100644
--- a/aom_mem/aom_mem.cmake
+++ b/aom_mem/aom_mem.cmake
@@ -13,9 +13,9 @@
 endif() # AOM_AOM_MEM_AOM_MEM_CMAKE_
 set(AOM_AOM_MEM_AOM_MEM_CMAKE_ 1)
 
-set(AOM_MEM_SOURCES "${AOM_ROOT}/aom_mem/aom_mem.c"
-    "${AOM_ROOT}/aom_mem/aom_mem.h"
-    "${AOM_ROOT}/aom_mem/include/aom_mem_intrnl.h")
+list(APPEND AOM_MEM_SOURCES "${AOM_ROOT}/aom_mem/aom_mem.c"
+            "${AOM_ROOT}/aom_mem/aom_mem.h"
+            "${AOM_ROOT}/aom_mem/include/aom_mem_intrnl.h")
 
 # Creates the aom_mem build target and makes libaom depend on it. The libaom
 # target must exist before this function is called.