perf util: Take elf_name as const string in dso__demangle_sym

The input string is not modified and thus can be passed in as a pointer
to const data.

Signed-off-by: Milian Wolff <milian.wolff@kdab.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Yao Jin <yao.jin@linux.intel.com>
Link: http://lkml.kernel.org/r/20170806212446.24925-3-milian.wolff@kdab.com


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