perf namespaces: Move the conditional setns() prototype to namespaces.h

Out of util.h, to reduce its scope, and since we have a namespaces.h
header, much better to have it there, where it is related to.

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-zlu81bbtccuzygh7m8nmgybc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>