perf namespaces: Remove namespaces.h from .h headers

There we need just forward declarations, so remove it and add it just on
the .c files that actually touch the struct definitions.

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-wsjxzt99p83jubt6hu0med0f@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/thread.h

@@ -13,6 +13,7 @@
 #include <intlist.h>
 #include "rwsem.h"
 
+struct namespaces_event;
 struct thread_stack;
 struct unwind_libunwind_ops;