blktrace: disable on PowerPC

This package has been failing to build on PowerPC since March 2016, with
nobody taking care of the problem, so let's disable this package on
PowerPC for the time being. PowerPC is the only architecture on which
blktrace fails to build currently.

Fixes:

  http://autobuild.buildroot.net/results/5a673c692aeb957cd8beb2c2c10ce5fd882ba4b5/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>