iio: stx104: Add X86 dependency to STX104 Kconfig option

iio: stx104: Add X86 dependency to STX104 Kconfig option

The Apex Embedded Systems STX104 DAC driver expects the STX104 device to
be I/O port-mapped addressable on an X86 platform. This patch adds an
explicit X86 dependency to the STX104 configuration option to prevent
building on non-X86 architectures.

Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>