perf unwind: Use 'struct map_symbol' in 'struct unwind_entry'

To help in passing that info around to callchain routines that, for the
same reason, are moving to use 'struct map_symbol'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-epsiibeprpxa8qpwji47uskc@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>