bpf: Export btf_type_by_id() and bpf_log()

These symbols will be used in nf_conntrack.ko to support direct writes
to `nf_conn`.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/3c98c19dc50d3b18ea5eca135b4fc3a5db036060.1662568410.git.dxu@dxuuu.xyz


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