hostfs: move HOSTFS_SUPER_MAGIC to <linux/magic.h>

Move HOSTFS_SUPER_MAGIC to <linux/magic.h> to be with it's magical
friends from other file systems.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>