Bump MIN_WINNT for MINGW to clear a build error

Bump MIN_WINNT for MINGW to clear a build error

Because we have been setting this too low, there has been a
long-standing warning about a missing declaration for inet_pton().
Modern gcc now considers this an error, so we have been getting failures
on the buildfarm animal fairywren.

Fix suggested by Thomas Munro.

This isn't needed in later branches, as they already set MIN_WINNT
higher, nor on earlier branches because they don't use inet_pton().

Discussion: https://postgr.es/m/574fae43-c993-4a25-b0e5-04c3e9c36d6d@dunslane.net