Remove meaningless const qualifier from ginCompressPostingList()

Hi Hacker,

While working on the other patch that fixed wrong "const" usage [1], I found the function:

```

GinPostingList *
ginCompressPostingList(const ItemPointer ipd, int nipd, int maxsize,
  int *nwritten)

```

uses "const" unnecessarily. Because it needs to assign an element of "ipd" to the returned structure "GinPostingList->first" and "first" is a mutable "ItemPointerData *", so that "ipd" cannot be of const pointer.

With the current usage, "const" only protects "ipd" itself from updating, which is meaningless. Thus the "const" only adds confusion to code readers, so I am deleting it.