perf symbol: Move C++ demangle defines to the only file using it

No need to have it generally available in such a critical header as
symbol.h.

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-es1ufxv7bihiumytn5dm3drn@git.kernel.org


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