Make the includes field an array in MSVC build scripts

Make the includes field an array in MSVC build scripts

Previously the 'includes' field was a string.  It's slightly nicer to
manage this when it's defined as an array instead. This allows us to
more easily detect and eliminate duplicates.

Reviewed-by: Álvaro Herrera, Andrew Dunstan, Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/CAApHDvpo6g5csCTjc_0C7DMvgFPomVb0Rh-AcW5afd=Ya=LRuw@mail.gmail.com