tools strfilter: Use __fallthrough

tools strfilter: Use __fallthrough

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

  util/strfilter.c: In function 'strfilter_node__sprint':
  util/strfilter.c:270:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
     if (len < 0)
        ^
  util/strfilter.c:272:2: note: here
    case '!':
    ^~~~
  cc1: all warnings being treated as errors

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-z2dpywg7u8fim000hjfbpyfm@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>