selftests/bpf: fix btf_dedup testing code

btf_dedup testing code doesn't account for length of struct btf_header
when calculating the start of a string section. This patch fixes this
problem.

Fixes: 49b57e0d ("tools/bpf: remove btf__get_strings() superseded by raw data API")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>