perf report: Show all sort keys in help output

Show all the supported sort keys in the command line help output, so
that it's not needed to refer to the manpage.

Before:

  % perf report -h
  ...
       -s, --sort <key[,key2...]>
                            sort by key(s): pid, comm, dso, symbol, parent, cpu, srcline, ... Please refer the man page for the complete list.

After:

  % perf report -h
  ...
      -s, --sort <key[,key2...]>
                            sort by key(s): overhead overhead_sys overhead_us overhead_guest_sys overhead_guest_us overhead_children sample period pid comm dso symbol parent cpu ...

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
LPU-Reference: 20190314225002.30108-5-andi@firstfloor.org
Link: https://lkml.kernel.org/n/tip-9r3uz2ch4izoi1uln3f889co@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>