win32: avoid discarding the exception handler

win32: avoid discarding the exception handler

In all likelihood, the compiler with lto doesn't see the function being
used, from assembly macro __try1. Help it by marking the function has
being used.

Resolves:
https://gitlab.com/qemu-project/qemu/-/issues/1904

Fixes: commit d89f30b4df ("win32: wrap socket close() with an exception handler")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>