perf c2c report: Add 'pid' sort key

It is to be displayed in the single cacheline output:

  pid

We currently don't have a single 'pid' sort/display entry, which would
output just pid number, hence adding it into c2c code.

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/n/tip-3o23qrspxc99b04ci1swlzr6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>