perf python: Fix undefined symbol problem

Recently we made perf_evsel__init call hists__init, which broke the perf
python binding:

[root@emilia linux]# ./tools/perf/python/twatch.py
Traceback (most recent call last):
  File "./tools/perf/python/twatch.py", line 16, in <module>
    import perf
ImportError: /home/acme/git/build/perf/python/perf.so: undefined symbol: hists__init

Fix it by moving the hists__init function to its only caller, evsel.c.

This way we avoid dragging in other parts of tools/perf/util/ to the
perf python binding.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-5nffmdt5mu6ozxgj54oi4qon@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/hist.h

@@ -63,8 +63,6 @@ struct hists {
 	struct callchain_cursor	callchain_cursor;
 };
 
-void hists__init(struct hists *hists);
-
 struct hist_entry *__hists__add_entry(struct hists *self,
 				      struct addr_location *al,
 				      struct symbol *parent, u64 period);