tools build: Add test for setns()

And provide an alternative implementation to keep perf building on older
distros as we're about to add initial support for namespaces.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-bqdwijunhjlvps1ardykhw1i@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/util.h

@@ -58,4 +58,8 @@ const char *perf_tip(const char *dirpath);
 int sched_getcpu(void);
 #endif
 
+#ifndef HAVE_SETNS_SUPPORT
+int setns(int fd, int nstype);
+#endif
+
 #endif /* GIT_COMPAT_UTIL_H */