perf evlist: Add missing 'struct option' forward declaration

It was being found, by chance, because evsel.h needlessly includes
util/cgroup.h, which will be sorted out in a following patch.

Cc: Adrian Hunter <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Don Zickus <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jean Pihet <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/util/evlist.h

@@ -117,6 +117,8 @@ int perf_evlist__prepare_workload(struct perf_evlist *evlist,
 						     void *ucontext));
 int perf_evlist__start_workload(struct perf_evlist *evlist);
 
+struct option;
+
 int perf_evlist__parse_mmap_pages(const struct option *opt,
 				  const char *str,
 				  int unset);