samples, bpf: Add tracex7 test file to .gitignore

This commit adds tracex7 test file (testfile.img) to .gitignore which
comes from test_override_return.sh.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200516040608.1377876-5-danieltimlee@gmail.com