tcf-agent: disable package for powerpc64

Fails to build, disable powerpc64 until upstream fixed it

Fixes:
  http://autobuild.buildroot.net/results/5088f6efd44ce9b081c2c7825a7005a9cb60799d



Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>