win/openssl: mark assembled object files as seh safe

There are no unsafe structured exception handlers in object files
generated from hand-crafted assembly - because they contain no exception
handlers at all.
Revert "build, windows: disable SEH"

This is no longer necessary - the underlying issue was fixed in 01fa5ee.
This reverts commit d87904286024f5ceb6a2d0d5f17e919c775830a0.