Fix compiler warning for hp backend

The debian-10-mini build generates the following warning:

   CC       libhp_la-hp-scl.lo
hp-scl.c: In function 'sanei_hp_nonscsi_new':
hp-scl.c:526:3: warning: 'strncpy' output truncated before terminating nul copying 36 bytes from a string of the same length [-Wstringop-truncation]
   strncpy ((char *)new->inq_data, "\003zzzzzzzHP      ------          R000",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            sizeof (new->inq_data));
            ~~~~~~~~~~~~~~~~~~~~~~~

This should be fixed. At first sight, this looks like it really should be a memcpy.