perf hists: Fix width computation for srcline sort entry

Adding header size to width computation for srcline sort entry,
because it's possible to get empty data with ':0' which set width
of 2 which is lower than width needed to display column header.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Joe Mario <jmario@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1474290610-23241-62-git-send-email-jolsa@kernel.org


[ Added declaration to sort.h ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>