bmc150_magn makes use of REGMAP_I2C, so select it to build always without
errors.

Fixes: c91746a2 ("iio: magn: Add support for BMC150 magnetometer")
Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>