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

samples/bpf/.gitignore

@@ -50,3 +50,4 @@ xdp_rxq_info
 xdp_sample_pkts
 xdp_tx_iptunnel
 xdpsock
+testfile.img