Make oom-heap.c test work with newer GCC (#1210)

New GCC versions can optimize the old version of the test to use
practically zero memory

cmstestsuite/code/oom-heap.c

 #include <stdio.h>
+#include <stdlib.h>
 
-int main() {
 int *big;
+
+int main() {
     int i;
     big = malloc(128 * 1024 * 1024);
     // If we don't do this cycle, the compiler is smart enough not to