Revert "kbx,w32: Disable the fd-passing."

This reverts commit 6944aefa3c2ef79cf3f14306ed384d22de36ba7f.

The fd-passing works well on Windows with new libassuan (to be 3.0),
and it doesn't require ASSUAN_SOCKET_SERVER_FDPASSING actually.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>