Protect <linux/console_struct.h> from multiple inclusion

Prevent <linux/console_struct.h> from being included more than once,
otherwise you get a redefinition error if you happen to include
<linux/vt_kern.h> first.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>