bpf: include errno.h from bpf-cgroup.h

Commit fdb5c453 ("bpf: fix attach type BPF_LIRC_MODE2 dependency
wrt CONFIG_CGROUP_BPF") caused some build issues, detected by 0-DAY
kernel test infrastructure.

The problem is that cgroup_bpf_prog_attach/detach/query() functions
can return -EINVAL error code, which is not defined. Fix this adding
errno.h to includes.

Fixes: fdb5c453 ("bpf: fix attach type BPF_LIRC_MODE2 dependency wrt CONFIG_CGROUP_BPF")
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Sean Young <sean@mess.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>