sensors: icm42688: ensure SENSOR_ASYNC_API is selected

This fix a build issue where ICM42688_DECODER is enabled but
SENSOR_ASYNC_API is not, which results in some structures in an
orphan section:

warning: orphan section
`._sensor_decoder_api.static.invensense_icm42688__decoder_api_' from
`libdrivers__sensor__icm42688.a(icm42688_decoder.c.obj)'
being placed in section
`._sensor_decoder_api.static.invensense_icm42688__decoder_api_'

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>