selftests/bpf: Select CONFIG_FUNCTION_ERROR_INJECTION

BPF selftests require CONFIG_FUNCTION_ERROR_INJECTION to work. However,
CONFIG_FUNCTION_ERROR_INJECTION is no longer 'y' by default after recent
changes. As a result, we are seeing errors like the following from BPF CI:

bpf_testmod_test_read() is not modifiable
__x64_sys_setdomainname is not sleepable
__x64_sys_getpgid is not sleepable

Fix this by explicitly selecting CONFIG_FUNCTION_ERROR_INJECTION in the
selftest config.

Fixes: a4412fdd49dc ("error-injection: Add prompt for function error injection")
Reported-by: Daniel Müller <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Daniel Müller <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/config

···
 CONFIG_DYNAMIC_FTRACE=y
 CONFIG_FPROBE=y
 CONFIG_FTRACE_SYSCALLS=y
+CONFIG_FUNCTION_ERROR_INJECTION=y
 CONFIG_FUNCTION_TRACER=y
 CONFIG_GENEVE=y
 CONFIG_IKCONFIG=y