bpf: Fix build without BPF_LSM.

resolve_btfids doesn't like empty set. Add unused ID when BPF_LSM is off.

Fixes: 1e6c62a ("bpf: Introduce sleepable BPF programs")
Reported-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Song Liu <songliubraving@fb.com>
Acked-by: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20200831163132.66521-1-alexei.starovoitov@gmail.com

kernel/bpf/verifier.c

@@ -11008,6 +11008,8 @@ BTF_SET_START(btf_sleepable_lsm_hooks)
 #ifdef CONFIG_BPF_LSM
 BTF_ID(func, bpf_lsm_file_mprotect)
 BTF_ID(func, bpf_lsm_bprm_committed_creds)
+#else
+BTF_ID_UNUSED
 #endif
 BTF_SET_END(btf_sleepable_lsm_hooks)