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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

samples/bpf/.gitignore

···
 xdp_sample_pkts
 xdp_tx_iptunnel
 xdpsock
+testfile.img