selftests/bpf: Xsk selftests - adding xdpxceiver to .gitignore

This patch adds *xdpxceiver* to selftests/bpf/.gitignore

Reported-by: Yonghong Song <yhs@fb.com>
Suggested-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Weqaar Janjua <weqaar.a.janjua@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201210115435.3995-1-weqaar.a.janjua@intel.com