perf top: Use __fallthrough

perf top: Use __fallthrough

The implicit fall through case label here is intended, so let us inform
that to gcc >= 7:

    CC       /tmp/build/perf/builtin-top.o
  builtin-top.c: In function 'display_thread':
  builtin-top.c:644:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
      if (errno == EINTR)
         ^
  builtin-top.c:647:3: note: here
     default:
   ^~~~~~~

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-lmcfnnyx9ic0m6j0aud98p4e@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>