Make the compiler not optimize my test out

Austin discovered it was broken with -c opt.

Change-Id: If2008f98b8079c9c40f8050400e3a250414d909f
diff --git a/build_tests/tcmalloc.cc b/build_tests/tcmalloc.cc
index c1d4429..7fb9105 100644
--- a/build_tests/tcmalloc.cc
+++ b/build_tests/tcmalloc.cc
@@ -1,6 +1,8 @@
 #include <stdlib.h>
+#include <unistd.h>
 
 int main() {
-  void *big_chunk = malloc(500000);
+  void *big_chunk = malloc(500000 + read(-1, nullptr, 0));
+  if (write(-1, big_chunk, 0) != -1) return 1;
   free(big_chunk);
 }