bpf: make LWTUNNEL_BPF dependent on INET

Lightweight tunnels are L3 constructs that are used with IP/IP6.

For example, lwtunnel_xmit is called from ip_output.c and
ip6_output.c only.

Make the dependency explicit at least for LWT-BPF, as now they
call into IP routing.

V2: added "Reported-by" below.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Oskolkov <posk@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>