selftests/bpf: Initialize duration in xdp_noinline.c

selftests/bpf: Initialize duration in xdp_noinline.c

Fixes clang error:
tools/testing/selftests/bpf/prog_tests/xdp_noinline.c:35:6: error: variable 'duration' is uninitialized when used here [-Werror,-Wuninitialized]
        if (CHECK(!skel, "skel_open_and_load", "failed\n"))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201001225440.1373233-1-sdf@google.com