Fix builds under other configurations

Change-Id: Ice6008b644b8a9d4879c007b760ba90b031ec511
diff --git a/build_tests/tcmalloc.cc b/build_tests/tcmalloc.cc
index 323ee58..eb74a65 100644
--- a/build_tests/tcmalloc.cc
+++ b/build_tests/tcmalloc.cc
@@ -1,3 +1,3 @@
 #include <stdlib.h>
 
-int main() { malloc(500000); }
+int main() { void *big_chunk __attribute__((unused)) = malloc(500000); }