selftests/bpf: add sign-file to .gitignore

Add sign-file to .gitignore to avoid accidentally checking it in.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-2-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/.gitignore

@@ -40,6 +40,7 @@ test_cpp
 /runqslower
 /bench
 /veristat
+/sign-file
 *.ko
 *.tmp
 xskxceiver