Given that C keeps a different namespace for `struct`s and aliases. The
following patterns
```c
typedef const struct foo {
    void *inner;
} *foo;

typedef struct bar {
    void *inner;
} *bar;
```
are valid C code and produces both a `struct` and a pointer called `foo`
and `bar` in different namespaces. Given that Rust does not make this
distinction, we add the `_ptr` prefix to the pointer type aliases to
avoid any name collisions.