selftests/bpf: Declare bpf_log_buf variables as static

The cgroup selftests did not declare the bpf_log_buf variable as static, leading
to a linker error with GCC 10 (which defaults to -fno-common). Fix this by
adding the missing static declarations.

Fixes: 257c8855 ("selftests/bpf: Convert test_cgroup_attach to prog_tests")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrey Ignatov <rdna@fb.com>
Link: https://lore.kernel.org/bpf/20200302145348.559177-1-toke@redhat.com