selftests/bpf: Remove duplicate include in test_lsm

'unistd.h' included in 'selftests/bpf/prog_tests/test_lsm.c' is
duplicated.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210105152047.6070-1-dong.menglong@zte.com.cn


Signed-off-by: Alexei Starovoitov <ast@kernel.org>