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.

index ed3408b..16184d6 100644
@@ -11,6 +11,7 @@
 #include <linux/vt_buffer.h>
 
 extern struct vc_data *sel_cons;
+struct tty_struct;
 
 extern void clear_selection(void);
 extern int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *tty);