libbpf: Add btf__new_split() API that was declared but not implemented

libbpf: Add btf__new_split() API that was declared but not implemented

Seems like original commit adding split BTF support intended to add
btf__new_split() API, and even declared it in libbpf.map, but never
added (trivial) implementation. Fix this.

Fixes: ba451366bf44 ("libbpf: Implement basic split BTF support")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240201172027.604869-4-andrii@kernel.org