iio: trigger: Add a blank line after declarations

This patch fixes the following checkpatch.pl warning:
WARNING: Missing a blank line after declarations

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>