libbpf: Fix parsing of multi-split BTF

libbpf: Fix parsing of multi-split BTF

When creating multi-split BTF we correctly set the start string offset
to be the size of the base string section plus the base BTF start
string offset; the latter is needed for multi-split BTF since the
offset is non-zero there.

Unfortunately the BTF parsing case needed that logic and it was
missed.

Fixes: 4e29128a9ace ("libbpf/btf: Fix string handling to support multi-split BTF")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251104203309.318429-2-alan.maguire@oracle.com