Fix alignment of GIN in-line posting lists stored in entry tuples.

The Sparc machines in the buildfarm are crashing because of misaligned
access to posting lists stored in entry tuples.

I accidentally removed a critical SHORTALIGN() from ginFormTuple, as part
of the packed posting lists patch. Perhaps I thought it was unnecessary,
because the index_form_tuple() call above the SHORTALIGN already aligned
the size, missing the fact that the null-category byte makes it misaligned
again (I think the SHORTALIGN is indeed unnecessary if there's no null-
category byte, but let's just play it safe...)