perf tools: Add header defining used namespace struct to event.h

perf tools: Add header defining used namespace struct to event.h

When adding the 'struct namespaces_event' to event.h, referencing the
'struct perf_ns_link_info' type, we forgot to add the header where it is
defined, getting that definition only by sheer luck.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Hari Bathini <hbathini@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: f3b3614a284d ("perf tools: Add PERF_RECORD_NAMESPACES to include namespaces related info")
Link: https://lkml.kernel.org/n/tip-qkrld0v7boc9uabjbd8csxux@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>