sadf: Don't test for activities available in file if only the header

sadf: Don't test for activities available in file if only the header
needs to be displayed

When "sadf -H" is used, there is no need to check that activities exist
in file. This is all the more true now as even CPU statistics (A_CPU
activity) don't necessarily exist in file.
E.g.:
sadc -S 0,36 datafile 1 2
Then "sadf -H datafile" should not display error message "Requested
activities not available in file".

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>