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: 1d56ade032a4 ("selftests/bpf: Unprivileged tests for test_loader.c")
Fixes: 0ab5539f8584 ("selftests/bpf: Tests for BPF_SK_LOOKUP attach point")
Fixes: 0905beec9f52 ("selftests/bpf: run flow dissector tests in skb-less mode")
Signed-off-by: Tony Ambardar <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/56643[e-mail]

tools/testing/selftests/bpf/prog_tests/flow_dissector.c

···
 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
 #include <network_helpers.h>
-#include <error.h>
 #include <linux/if_tun.h>
 #include <sys/uio.h>
 

tools/testing/selftests/bpf/prog_tests/sk_lookup.c

···
 #include <arpa/inet.h>
 #include <assert.h>
 #include <errno.h>
-#include <error.h>
 #include <fcntl.h>
 #include <sched.h>
 #include <stdio.h>

tools/testing/selftests/bpf/unpriv_helpers.c

···
 
 #include <stdbool.h>
 #include <stdlib.h>
-#include <error.h>
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>