AutoMoc: Only add the WIN32 define if the target system is Windows

When there's no moc_predefs.h generated, we add the WIN32 define as
fallback.

Before this change, we added the WIN32 define if the host system was
Windows.  That's wrong when cross-compiling.  Now we're checking whether
the target system is Windows.

Fixes: #22933
43 jobs for !6751 with fix-22933 in 23 minutes and 10 seconds (queued for 1 second)
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