Now that thread.h isn't included by any other header, we can check where
it is really needed, i.e. we can remove it and be sure that it isn't
being obtained indirectly.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-kh333ivjbw05wsggckpziu86@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>