perf diff: Move columns into struct data__file

perf diff: Move columns into struct data__file

Another step towards multiple data files support. Having columns
definition within struct data__file force each data file having its own
columns.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-lnfqj7k7fqw8bz07pupi5464@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>