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

Change-Id: Ia7df73b5cd8f242228bc4f8dacff8212600fa7ff

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom