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 <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-xsvxr747wkkpg1ay9dramorr@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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);