perf tools: Add missing struct definition in probe_event.h

Commit 0b3c2264 ("perf symbols: Fix kallsyms perf test on ppc64le")
refers struct symbol in probe_event.h, but forgets to include its
definition.  Gcc will complain about it when that definition is not
added, by sheer luck, by some other header included before
probe_event.h.
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/20161115040617.69788-4-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.h

@@ -18,6 +18,8 @@ struct probe_conf {
 extern struct probe_conf probe_conf;
 extern bool probe_event_dry_run;
 
+struct symbol;
+
 /* kprobe-tracer and uprobe-tracer tracing point */
 struct probe_trace_point {
 	char		*realname;	/* function real name (if needed) */