selftests/bpf: drop custom NULL #define in skb_pkt_end selftest

Now that bpftool generates NULL definition as part of vmlinux.h, drop custom
NULL definition in skb_pkt_end.c.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20210317200510.1354627-3-andrii@kernel.org


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