iio:light:ltr501: fix regmap dependency

The use of regmap in commit 2f2c9633 requires REGMAP_I2C to be selected, in
order to meet all dependencies.

Fixes: 2f2c9633 ("iio: ltr501: Add regmap support.")

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>