Minimal portability fix for commit e1551f96e.

Minimal portability fix for commit e1551f96e.

Older gcc versions are not happy with having multiple declarations
for the same typedef name (not struct name).  I'm a bit dubious
as to how well-thought-out that patch was at all, but for the moment
just fix it enough so I can get some work done today.

Discussion: https://postgr.es/m/20191218101338.GB325369@paquier.xyz