dtc: fix sprintf() format string error, again

2a42b14d "dtc: check.c fix compile error" changed a format string using
%lx which wasn't correct for all platforms.  Unfortunately it changed it to
%zx, which is wrong for a different set of platforms (and only right on
the others by accident).  The parameter we're formatting here is uint64_t,
not size_t, so we need to use the PRIx64 macro from <inttypes.h> to get
this right.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>