Fix duplicate typedef from commit 0d8c9c121.

Fix duplicate typedef from commit 0d8c9c121.

Older gcc versions don't like duplicate typedefs, so get rid of
that in favor of doing it like we do it elsewhere, ie just use
a "struct" declaration when trying to avoid importing a whole
header file.

Also, there seems no reason to include stringinfo.h here at all,
so get rid of that addition too.

Discussion: https://postgr.es/m/27239.1587415696@sss.pgh.pa.us