iio: always select ANON_INODES

Without ANON_INODES, we get this build error:

drivers/built-in.o: In function `iio_event_getfd':
:(.text+0x14bf18): undefined reference to `anon_inode_getfd'

All other users explicitly select this symbol, so we should
do the same thing here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>