selftests: bpf: fix -Wformat-security warning for flow_dissector_load.c

flow_dissector_load.c:55:19: warning: format string is not a string literal (potentially insecure)
      [-Wformat-security]
                error(1, errno, command);
                                ^~~~~~~
flow_dissector_load.c:55:19: note: treat the string as an argument to avoid this
                error(1, errno, command);
                                ^
                                "%s",
1 warning generated.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>