input: hbtp_input: NULL terminate list of attributes

The attrs member of struct attribute_group should be a pointer
to NULL terminated list of struct attribute. NULL terminate
the declaration of variable secure_touch_attrs.

Change-Id: I970e31dc1ae56a05ce172b23b49a5471bec77cce
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>