samples: Build hts221 sensor sample only on supported boards

The hts221 sensor driver was implicitly made dependent upon the
disco_l475_iot1 board when it was converted to dts, so build the
associated sample only for this board. This issue was encountered when
adding sensor dts support to nxp boards, which don't have hts221.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>
2 files changed