selftests/powerpc: update .gitignore

I did an in-place build of the self-tests and found that it left
the tree dirty.

Add missed test binaries to .gitignore

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201201144427.1228745-1-dja@axtens.net