[ Reason ]
The glibc version 2.41-12+deb13u4 upload to the 13.7 point release
stopped including <fcntl.h> from <sys/mount.h>, in order to avoid
namespace conflicts with linux 7.0+ headers. This causes the FTBFS of
switchsh, which does not directly include <fcntl.h> while using the
open() function.

[ Impact ]
The package now FTBFS in trixie starting with the 13.7 point release.
The binary package still works fine.

[ Tests ]
I used sbuild to confirm that the package build fine again in trixie.

[ Risks ]
There is a one line code change, basically adding an extra <fcntl.h>
include that was included indirectly through <sys/mount.h> before. The
same change has been applied to testing/unstable already. Therefore the
risk is close to zero.

[ Checklist ]
  [x] *all* changes are documented in the d/changelog
  [x] I reviewed all changes and I approve them
  [x] attach debdiff against the package in (old)stable
  [x] the issue is verified as fixed in unstable

[ Changes ]
The change consists adding the open.patch file from unstable. This patch
just adds an extra <fcntl.h> to declare, among other things, the open()
function.

[ Other info ]
Given the change are trivial, I have already uploaded the package to the
archive.