autofs4: fix string.h include in auto_dev-ioctl.h

Since including linux/string.h will now do the right thing remove the
conditional check.

Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>