staging:iio:ad7298: Fix linker error due to missing IIO kfifo buffer

The ad7298 drivers buffer implementation uses the IIO kfifo buffer, so it needs
to select IIO_KFIFO_BUF. Otherwise (if no other driver selects the symbol) the
following linker error will occur:

drivers/built-in.o: In function `ad7298_register_ring_funcs_and_init':
	(.text+0x245cf2): undefined reference to `iio_kfifo_allocate'
drivers/built-in.o: In function `ad7298_register_ring_funcs_and_init':
	(.text+0x245d7d): undefined reference to `iio_kfifo_free'
drivers/built-in.o: In function `ad7298_ring_cleanup':
	(.text+0x245dcd): undefined reference to `iio_kfifo_free'

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>