perf hists browser: Increase size of 'buf' in perf_evsel__hists_browse()

Making perf with gcc-9.1.1 generates the following warning:

    CC       ui/browsers/hists.o
  ui/browsers/hists.c: In function 'perf_evsel__hists_browse':
  ui/browsers/hists.c:3078:61: error: '%d' directive output may be \
  truncated writing between 1 and 11 bytes into a region of size \
  between 2 and 12 [-Werror=format-truncation=]

   3078 |       "Max event group index to sort is %d (index from 0 to %d)",
        |                                                             ^~
  ui/browsers/hists.c:3078:7: note: directive argument in the range [-2147483648, 8]
   3078 |       "Max event group index to sort is %d (index from 0 to %d)",
        |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from /usr/include/stdio.h:937,
                   from ui/browsers/hists.c:5:

IOW, the string in line 3078 might be too long for buf[] of 64 bytes.

Fix this by increasing the size of buf[] to 128.

Fixes: dbddf174

  ("perf report/top TUI: Support hotkeys to let user select any event for sorting")
Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: stable@vger.kernel.org # v5.7+
Link: http://lore.kernel.org/lkml/20201030235431.534417-1-songliubraving@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>