tty: vt: make vc_is_sel()'s vc const

It's only an aid to people reading the header and/or calling
vc_is_sel(). vc is only tested there, so having it const makes sense.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Tested-by: Helge Deller <deller@gmx.de> # parisc STI console
Link: https://lore.kernel.org/r/20240122110401.7289-9-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>