perf tools: No need to include namespaces.h in util.h

perf tools: No need to include namespaces.h in util.h

The only thing that is needed there is a forward declaration for 'struct
nsinfo', so disentanble this, which in turns allows built-in clang
builds, i.e. 'make LIBCLANGLLVM=1 -C tools/perf'.

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: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Cc: Sandipan Das <sandipan@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-vq26rsuwq1cqylpcyvq89c84@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>