Remove xdp_samples_user.o rule redefinition which generates Makefile
warning and instead override TPROGS_CFLAGS. This seems to work fine when
building inside selftests/bpf.

That was one big head-scratcher before I found that generic
Makefile.target hid this surprising specialization for for xdp_samples_user.o.

Main change is to use actual locally installed libbpf headers.

Also drop printk macro re-definition (not even used!).
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211201232824.3166325-8-andrii@kernel.org