perf tools: Remove extra '/' character in events file path

The array debugfs_known_mountpoints[] will cause extra '/'
character output.
Remove it.

pre:
$ perf probe -l
/sys/kernel/debug//tracing/uprobe_events file does not exist -
please rebuild kernel with CONFIG_UPROBE_EVENTS.

post:
$ perf probe -l
/sys/kernel/debug/tracing/uprobe_events file does not exist -
please rebuild kernel with CONFIG_UPROBE_EVENTS.

Signed-off-by: Xia Kaixu <xiakaixu@huawei.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/535B6660.2060001@huawei.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>