perf symbols: Introduce map_symbol.h

To allow headers just wanting this definition to be able to get it
without all the things in symbol.h, to reduce the include dep tree.

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-l32z2qyhs6fe8unf4gk2ead2@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>