perf data: Rename directory "header" file to "data"

perf data: Rename directory "header" file to "data"

In preparation to support a single file directory format, rename "header"
to "data" because "header" is a mis-leading name when there is only 1 file.
Note, in the multi-file case, the "header" file also contains data.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20191004083121.12182-4-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>