os.h: change elif _WIN32 to elif defined(_WIN32)
os.h: change elif _WIN32 to elif defined(_WIN32)

This symbol is only defined (with the value 1) when building
for the Windows target, so we need to ifdef, not if.

Signed-off-by: Ralph Giles <65585f72bf62a11d2f4105aa30c20fe94efea46f@thaumas.net>

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