perf symbols: Move symbol_conf to separate file

So that we don't drag all the headers included in symbol.h when needing
to access symbol_conf in another header, such as annotate.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-rvo9dzflkneqmprb0dgbfybx@git.kernel.org


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