perf intel-pt: Use __fallthrough

perf intel-pt: Use __fallthrough

To address new warnings emmited by gcc 7, e.g.::

    CC       /tmp/build/perf/util/intel-pt-decoder/intel-pt-pkt-decoder.o
    CC       /tmp/build/perf/tests/parse-events.o
  util/intel-pt-decoder/intel-pt-pkt-decoder.c: In function 'intel_pt_pkt_desc':
  util/intel-pt-decoder/intel-pt-pkt-decoder.c:499:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
     if (!(packet->count))
        ^
  util/intel-pt-decoder/intel-pt-pkt-decoder.c:501:2: note: here
    case INTEL_PT_CYC:
    ^~~~
    CC       /tmp/build/perf/util/intel-pt-decoder/intel-pt-decoder.o
  cc1: all warnings being treated as errors

Acked-by: Andi Kleen <ak@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.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-mf0hw789pu9x855us5l32c83@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>