selftests/bpf: Enable CONFIG_VSOCKETS in config

CONFIG_VSOCKETS is required by BPF selftests, otherwise we get errors
like this:

    ./test_progs:socket_loopback_reuseport:386: socket:
		Address family not supported by protocol
    socket_loopback_reuseport:FAIL:386
    ./test_progs:vsock_unix_redir_connectible:1496:
		vsock_socketpair_connectible() failed
    vsock_unix_redir_connectible:FAIL:1496

So this patch enables it in tools/testing/selftests/bpf/config.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Link: https://lore.kernel.org/r/472e73d285db2ea59aca9bbb95eb5d4048327588.1696490003.git.geliang.tang@suse.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>