perf diff: Fix duplicated output column

The commit b97511c5bc94 ("perf tools: Add overhead/overhead_children
keys defaults via string") moved initialization of column headers but it
missed to check the sort__mode.  As 'perf diff' doesn't call
perf_hpp__init(), the setup_overhead() also should not be called.

Before:

  # Baseline    Delta  Children  Overhead  Shared Object        Symbol
  # ........  .......  ........  ........  ...................  .......................
  #
      28.48%  -28.47%    28.48%    28.48%  [kernel.vmlinux ]    [k] intel_idle
      11.51%  -11.47%    11.51%    11.51%  libxul.so            [.] 0x0000000001a360f7
       3.49%   -3.49%     3.49%     3.49%  [kernel.vmlinux]     [k] generic_exec_single
       2.91%   -2.89%     2.91%     2.91%  libdbus-1.so.3.8.11  [.] 0x000000000000cdc2
       2.86%   -2.85%     2.86%     2.86%  libxcb.so.1.1.0      [.] 0x000000000000c890
       2.44%   -2.39%     2.44%     2.44%  [kernel.vmlinux]     [k] perf_event_aux_ctx

After:

  # Baseline    Delta  Shared Object        Symbol
  # ........  .......  ...................  .......................
  #
      28.48%  -28.47%  [kernel.vmlinux]     [k] intel_idle
      11.51%  -11.47%  libxul.so            [.] 0x0000000001a360f7
       3.49%   -3.49%  [kernel.vmlinux]     [k] generic_exec_single
       2.91%   -2.89%  libdbus-1.so.3.8.11  [.] 0x000000000000cdc2
       2.86%   -2.85%  libxcb.so.1.1.0      [.] 0x000000000000c890
       2.44%   -2.39%  [kernel.vmlinux]     [k] perf_event_aux_ctx

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: <stable@vger.kernel.org> # 4.5+
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: b97511c5bc94 ("perf tools: Add overhead/overhead_children keys defaults via string")
Link: http://lkml.kernel.org/r/1462890384-12486-2-git-send-email-acme@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>