When CONFIG_IIO_TRIGGER is enabled but CONFIG_IIO_BUFFER is
not, we get a build error in the st_magn driver:
drivers/iio/magnetometer/st_magn_core.c:573:23: error: 'ST_MAGN_TRIGGER_SET_STATE' undeclared here (not in a function)
.set_trigger_state = ST_MAGN_TRIGGER_SET_STATE,
^~~~~~~~~~~~~~~~~~~~~~~~~
Apparently, this ST_MAGN_TRIGGER_SET_STATE macro was meant to
be set to NULL when the definition is not available because
st_magn_buffer.c is not compiled, but the alternative definition
was not included in the original patch. This adds it.