[cxx-interop] Update test for foreign reference types

On Linux, both the C++ stdlib (libstdc++) and the C stdlib (glibc) have a header called `stdlib.h`.

This test is failing while I am restructuring Glibc/libstdc++ modulemaps on Linux. Besides, the tests that use the C++ stdlib reside in the `Cxx/stdlib` directory.

Let's fix that by including the C header and not the C++ header.

Author: egorzhdan

test/Interop/Cxx/foreign-reference/Inputs/move-only.h

@@ -1,10 +1,11 @@
 #ifndef TEST_INTEROP_CXX_FOREIGN_REFERENCE_INPUTS_MOVE_ONLY_H
 #define TEST_INTEROP_CXX_FOREIGN_REFERENCE_INPUTS_MOVE_ONLY_H
 
-#include <stdlib.h>
+#include <stddef.h> // for size_t
 
 #include "visibility.h"
 
+void* malloc(size_t size);
 inline void *operator new(size_t, void *p) { return p; }
 
 template <class _Tp>