tools string: Use __fallthrough in perf_atoll()

tools string: Use __fallthrough in perf_atoll()

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

    CC       /tmp/build/perf/util/string.o
  util/string.c: In function 'perf_atoll':
  util/string.c:22:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
      if (*p)
         ^
  util/string.c:24:3: note: here
     case '\0':
     ^~~~

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-0ophb30v9apkk6o95el0rqlq@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>