[flang][OMP] change malloc.h to stdlib.h in collapse_test.inc (NFC) (llvm#122711)

Author: kkwli

openmp/runtime/test/worksharing/for/collapse_test.inc

@@ -1,5 +1,5 @@
 #include <omp.h>
-#include <malloc.h>
+#include <stdlib.h>
 #include <stdio.h>
 #include <memory.h>