perf tools: Don't add kernel directories to the header search path

perf tools: Don't add kernel directories to the header search path

We've decided not to access kernel source files because changes there
could break the tooling side, this is one more step in that direction.

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-ar0hupkxl45h5hk09l2rprj3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>