selftests: Add .gitignore for nci test suite

Building the nci test suite produces a binary, nci_dev, that git then
tries to track. Add a .gitignore file to tell git to ignore this binary.
Signed-off-by: NDavid Matlack <dmatlack@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/nci/.gitignore

+/nci_dev