perf hists: Fix typo on hist__entry_add_pair

perf hists: Fix typo on hist__entry_add_pair

Fix a misplaced underscore.  In this case, 'hist_entry' is the name of
data structure and we usually put double underscores between data
structure and actual function name.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>,
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-8jdq8g6kl6v54hkexrfwsy72@git.kernel.org
[ committer note: put it in front of the patch queue where it came from ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>