perf dso: Export data_file_size() method there are no symbols

Will be used outside dso.c in a followup patch, so rename it and make it
non-static.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181127084634.12469-1-adrian.hunter@intel.com


[ split from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>