Fix error building on Clang on windows

[Rageoholic]

Clang apparently doesn't implement strdup in the version of libc they
ship. Instead they implement _strdup which is identical. This change
just makes it so that on windows we just call _strdup in yaml_strdup.

[jeremyevans]

This patch does not appear to actually change anything, as the ifdef and else sections are identical. Based on the commit message, I think the intention is to use _strdup instead of strdup in the ifdef section.