perf diff: color the Weighted Diff column

In

  $ perf diff -c wdiff:M,N

color the numbers in the Weighted Diff column using
value_color_snprintf().

Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>