Fix malloc declaration in Linked_List1 regression test

We had invalid pointers instead of dynamic objects in this test, which
didn't matter to much. It may, however, matter with upcoming support for
non-deterministic pointers.

Author: tautschnig

regression/cbmc/Linked_List1/main.c

@@ -1,4 +1,4 @@
-void *malloc(unsigned int s);
+void *malloc(__CPROVER_size_t s);
 
 struct nodet
 {