vt: Fix warnings in selection.h

<linux/selection.h> assumes that struct tty_struct has previously been
included.  If not, this pile of warnings will result:

  CC [M]  drivers/video/console/newport_con.o
In file included from drivers/video/console/newport_con.c:18:
include/linux/selection.h:16: warning: 'struct tty_struct' declared inside param
eter list
include/linux/selection.h:16: warning: its scope is only this definition or decl
aration, which is probably not what you want
include/linux/selection.h:17: warning: 'struct tty_struct' declared inside param
eter list
include/linux/selection.h:20: warning: 'struct tty_struct' declared inside param
eter list

Fixed by adding a forward declaration of struct tty_struct.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>