perf annotate: Add --skip-empty option

Like in perf report, we want to hide empty events in the perf annotate
output.  This is consistent when the option is set in perf report.

For example, the following command would use 3 events including dummy.

  $ perf mem record -a -- perf test -w noploop

  $ perf evlist
  cpu/mem-loads,ldlat=30/P
  cpu/mem-stores/P
  dummy:u

Just using perf annotate with --group will show the all 3 events.

  $ perf annotate --group --stdio | head
   Percent                 |	Source code & Disassembly of ...
  --------------------------------------------------------------
                           : 0     0xe060 <_dl_relocate_object>:
      0.00    0.00    0.00 :    e060:       pushq   %rbp
      0.00    0.00    0.00 :    e061:       movq    %rsp, %rbp
      0.00    0.00    0.00 :    e064:       pushq   %r15
      0.00    0.00    0.00 :    e066:       movq    %rdi, %r15
      0.00    0.00    0.00 :    e069:       pushq   %r14
      0.00    0.00    0.00 :    e06b:       pushq   %r13
      0.00    0.00    0.00 :    e06d:       movl    %edx, %r13d

Now with --skip-empty, it'll hide the last dummy event.

  $ perf annotate --group --stdio --skip-empty | head
   Percent         |	Source code & Disassembly of ...
  ------------------------------------------------------
                   : 0     0xe060 <_dl_relocate_object>:
      0.00    0.00 :    e060:       pushq   %rbp
      0.00    0.00 :    e061:       movq    %rsp, %rbp
      0.00    0.00 :    e064:       pushq   %r15
      0.00    0.00 :    e066:       movq    %rdi, %r15
      0.00    0.00 :    e069:       pushq   %r14
      0.00    0.00 :    e06b:       pushq   %r13
      0.00    0.00 :    e06d:       movl    %edx, %r13d

Signed-off-by: Namhyung Kim <namhyung@kernel.org>