[flang] change malloc.h to stdlib.h (NFC)

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>