perf symbols: Introduce method to iterate symbols ordered by name

Given a symbol, go to the next entry in a rbtree sorted by symbol name.

Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Link: http://lkml.kernel.org/n/tip-aq210drxprnu2so4dye5xa3j@git.kernel.org


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