tools: Adopt __packed from kernel sources

To have a more compact way to ask the compiler to not insert alignment
paddings in a struct, making tools/ look more like kernel source code.

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-byp46nr7hsxvvyc9oupfb40q@git.kernel.org


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