Fix for msbuild/mingw GHA failures

Fix for msbuild/mingw GHA failures

Commit cd8e25a6e9 removed a variable because it looked as if
used only once anyway (1 assignment, 1 usage) - overlooking that
on _WIN32 it's changed to NULL, which wasn't adjusted...

This fix restores the wiped out "unsigned int *persistent" in
multi_tcp_wait(), undoing this particular change of the previous
commit.

Change-Id: I8526aadb5151ddc997c836d5a691bcdfee700938
Signed-off-by: Gianmarco De Gregori <gianmarco@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20241023113923.7420-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29612.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>