selftests/bpf: Enable CONFIG_IKCONFIG_PROC in config

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

libbpf: failed to open system Kconfig
libbpf: failed to load object 'kprobe_multi'
libbpf: failed to load BPF skeleton 'kprobe_multi': -22

It's because /proc/config.gz is opened in bpf_object__read_kconfig_file()
in tools/lib/bpf/libbpf.c:

file = gzopen("/proc/config.gz", "r");

So this patch enables CONFIG_IKCONFIG and CONFIG_IKCONFIG_PROC in
tools/testing/selftests/bpf/config.

Suggested-by: Mat Martineau <[e-mail]>
Signed-off-by: Geliang Tang <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/config

···
 CONFIG_NF_CONNTRACK=y
 CONFIG_USERFAULTFD=y
 CONFIG_FPROBE=y
+CONFIG_IKCONFIG=y
+CONFIG_IKCONFIG_PROC=y