sysdeps: Stop pretending _dbus_set_signal_handler is portable to Windows

sysdeps: Stop pretending _dbus_set_signal_handler is portable to Windows

None of the things we rely on in POSIX async signal handlers, such
as the existence of async-signal-safe write(), are portable to Windows,
so the async signal handlers that use this function are #ifdef
DBUS_UNIX anyway. Remove the unused stub function from the
Windows side, and move the declaration to the Unix-specific header.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=103010
Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>