selftests/bpf: Enable CONFIG_FPROBE for self tests

Some of the BPF selftests are failing when running with a rather bare
bones configuration based on tools/testing/selftests/bpf/config.
Specifically, we see a bunch of failures due to errno 95:

  > test_attach_api:PASS:fentry_raw_skel_load 0 nsec
  > libbpf: prog 'test_kprobe_manual': failed to attach: Operation not supported
  > test_attach_api:FAIL:bpf_program__attach_kprobe_multi_opts unexpected error: -95
  > 79 /6     kprobe_multi_test/attach_api_syms:FAIL

The cause of these is that CONFIG_FPROBE is missing. With this change we
add this configuration value to the BPF selftests config.

Signed-off-by: Daniel Müller <deso@posteo.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20220511172249.4082510-1-deso@posteo.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>