selftests/bpf: Drop unneeded error.h includes

The addition of general support for unprivileged tests in test_loader.c
breaks building test_verifier on non-glibc (e.g. musl) systems, due to the
inclusion of glibc extension '<error.h>' in 'unpriv_helpers.c'. However,
the header is actually not needed, so remove it to restore building.

Similarly for sk_lookup.c and flow_dissector.c, error.h is not necessary
and causes problems, so drop them.

Fixes: 1d56ade0 ("selftests/bpf: Unprivileged tests for test_loader.c")
Fixes: 0ab5539f ("selftests/bpf: Tests for BPF_SK_LOOKUP attach point")
Fixes: 0905beec ("selftests/bpf: run flow dissector tests in skb-less mode")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/5664367edf5fea4f3f4b4aec3b182bcfc6edff9c.1721713597.git.tony.ambardar@gmail.com