libbpf: Fix huge memory leak in libbpf_find_vmlinux_btf_id()

BTF object wasn't freed.

Fixes: a6ed02ca ("libbpf: Load btf_vmlinux only once per object.")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20200429012111.277390-9-andriin@fb.com