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>