tty: vt: remove extern from functions in selection.h

Remove unneeded 'extern' keyword from function prototypes in
selection.h. This makes it more readable as no more wrapping is needed
on many places.

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/20240122110401.7289-19-jirislaby@kernel.org


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