tools lib subcmd: Add missing linux/kernel.h include to subcmd.h

As it was getting the BUILD_BUG_ON_ZERO() definition by luck.

Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-dh71o31ar72ajck8o2x4aoae@git.kernel.org


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