iio:configfs: Introduce iio/configfs.h to provide a location for the configfs_subsystem

This exported element needs to be accesible to all drivers using configfs
within IIO.  Previously it was in the sw_trig.h file which only convered one
such usecase.  This also fixes a sparse warning as it is now in a header
that makes sense to include from industrialio-configfs.c

Signed-off-by: Jonathan Cameron &lt; <jic23@kernel.org>