perf hists: Fix width computation for srcline sort entry

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 <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Don Zickus <[email protected]>
Cc: Joe Mario <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
[ Added declaration to sort.h ]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>