If 'struct rq' isn't defined in lock_contention.bpf.c then the type for
the 'runqueue' variable ends up being a forward declaration
(BTF_KIND_FWD) while the kernel has it defined (BTF_KIND_STRUCT).

This makes libbpf decide it has incompatible types and then fails to
load the BPF skeleton:

  # perf lock con -ab sleep 1
  libbpf: extern (var ksym) 'runqueues': incompatible types, expected [95] fwd rq, but kernel has [55509] struct rq
  libbpf: failed to load object 'lock_contention_bpf'
  libbpf: failed to load BPF skeleton 'lock_contention_bpf': -22
  Failed to load lock-contention BPF skeleton
  lock contention BPF setup failed
  #

Add it as an empty struct to satisfy that type verification:

  # perf lock con -ab sleep 1
   contended   total wait     max wait     avg wait         type   caller

           2     50.64 us     25.38 us     25.32 us     spinlock   tick_do_update_jiffies64+0x25
           1     26.18 us     26.18 us     26.18 us     spinlock   tick_do_update_jiffies64+0x25
  #

Committer notes:

Extracted from a larger patch as Namhyung had already fixed the other
issues in e53de7b6 ("perf lock contention: Fix struct rq lock
access").
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Changbin Du <changbin.du@huawei.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Link: https://lore.kernel.org/lkml/ZFVqeKLssg7uzxzI@kravaSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>