perf hists browser: Add back callchain folding symbol

The commit 5395a048

 ("perf hists: Separate overhead and baseline
columns") makes the "Overhead" column no more the first one, this
caused the test that checks if it is time to show if a histogram
entry has callchains never hits.

Fix it by checking if the 'i' variable is equal to PERF_HPP__OVERHEAD
instead of 0.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
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-w3lcbx0fx1fnh3l2cbq40q2e@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>